Method of joining metal pieces

ABSTRACT

A method and apparatus for joining metal pieces wherein the rear end of a preceding metal piece and a front end of a metal piece are heated and then pressed together prior to hot finished rolling. The temperature the ends of the metal pieces are heated to is a temperature range between the solidus and liquidus line temperatures of the metal(s) to be joined.

This is a divisional of application U.S. Ser. No. 08/513,789 filed onDec. 29, 1995 under 35 USC §371, U.S. Pat No. 5,951,903.

TECHNICAL FIELD

The present invention relates to a metal piece joining method useful forcontinuous hot rolling, by which a preceding metal piece and asucceeding metal piece are heated and joined with each other at theirends to continuously carry out hot finishing rolling, and also to ajoining apparatus which is directly used for carrying out the method.

BACKGROUND ART

Conventionally, in a hot rolling line for metal pieces (for example,steel, aluminum or copper), since the metal pieces have been extractedone by one from a heating furnace, there occurred various problemsparticularly in the finishing rolling process, as follows.

1) A biting failure at the front end of a metal piece.

2) A defective stamping at the rear end of a metal piece (a phenomenonthat a corner at the rear end of a thin metal piece is bent).

3) A traveling problem of the front end of a metal piece occurring on arun-out table.

4) A defective dimension at the front and rear ends of metal pieces.

As means for solving the above-mentioned problems, there has beenproposed a so-called endless rolling in which metal pieces to be rolledare joined at their rear and front ends before the finishing rolling,and continuously supplied to the fomoshinh rolling line to carry out thehot rolling.

As prior documents in this regard, a number of propositions can befound, for example, in Japanese Patent Laid-open Publication Nos.60-244401, 61-144203, 62-234679, 4-89109, 4-89115 and 4-89110.

When carrying out the endless rolling of metal pieces, the followingprocess has been generally performed. That is, firstly, on the entryside of a rolling equipment, a small gap is provided between a precedingmetal piece and a succeeding metal piece at the ends thereof and thesemetal pieces are opposed substantially in parallel with each other.Further, a portion in the vicinity of the end of each metal piece isclamped and supported by clamps, and the end regions on the opposedfaces of the metal pieces, which are portions to be joined, are heatedby a heating means. The both metal pieces are then pressed against eachother to be joined. In the joining method for metal pieces using such aprocess, various disadvantages which will be described below have stillremained and an improvement in this regard has been desired.

1) In the joining form of this type, there is provided an inductorhaving a pair of magnetic poles vertically sandwiching the portions tobe joined in the metal pieces. With the inductor, an alternatingmagnetic field running through the metal pieces in the thicknessdirection thereof is applied, and a surface layer at the portions to bejoined, or on the opposed surfaces in particular is intensively heatedby the induced current generated at this time. The induced current is,however, hard to flow at corners of the front and rear ends of the metalpieces, and the heating temperature at the portions to be joined thusgradually lowers toward the wide ends. Thus, there is such adisadvantage that the portions to be joined cannot be joined over thefull width thereof when pressing the metal pieces against each other.

In this case, the wide ends having a low temperature function as aresistance when pressing the metal pieces against each other, if thetemperature at the portions to be joined does not reach a target heatingtemperature, so that a pressing apparatus having a capacity above anecessary level must be installed. Also, since a sufficient joiningstrength cannot be secured, the joined portions are gradually separatedas the rolling proceeds and the metal plate are ruptured so that aserious accident may occur. In order to solve such a problem, it is mosteffective to continue the heating until the temperature at the cornersof the steel pieces reaches a target value. However, since thetemperature in regions other than the corners (i.e., central regions inthe plate width direction) reaches a melting temperature and the metalpieces are melted down in these regions, excellent joined portionscannot be obtained. In addition, such a heating may deteriorate thesurface quality of the plate at the portions after rolling whichcorrespond to the joining portions or other portions close thereto, andthe input electrical power has to be increased as well.

2) In the induction heating method using the inductor, since a variationrate of magnetic fluxes (a variation rate of the number of magneticfluxes) is proportional to the current to be induced, the variation ratebecomes large as the number of magnetic fluxes running through the metalpieces is large at the peak of the alternating current which flows in acoil of the inductor, thus increasing the scale of the current. Further,since a vertical component in the magnetic flux generated by theinductor for the surface of the magnetic pieces advantageouslycontributes to the generation of the induced current, the inducedcurrent can be increased as the magnetic flux running through themagnetic pieces becomes vertical.

However, in the metal piece heating and joining to which the inductionheating method is applied, if the positional relationship between theinductor and the ends of the respective metal pieces (the position ofthe inductor in the longitudinal direction of the metal pieces inparticular) is inadequate, the number of magnetic fluxes running throughthe metal pieces is insufficient and the vertical component of themagnetic flux is also disadvantageously reduced. Further, for example,in a conventional method disclosed in Japanese Patent Laid-openPublication No. 62-234679 mentioned above, this kind of disadvantage isnot considered therein, and the satisfactory heating speed cannot beobtained depending on the positional relationship between the inductorand each metal piece, involving a prolongation of the heating time.Also, since the preceding metal piece and the succeeding metal piece arenot uniformly and equally heated, an excellent joining cannot berealized.

3) When heating the metal pieces, if the preceding metal piece and thesucceeding metal piece are different in the initial temperature or inthe plate thickness, or if the metal pieces having different meltingpoints are heated, an appropriate heating cannot be performed inaccordance with each metal piece, and a sufficient strength cannot begiven to the joined metal pieces. In such a case, the plates may beruptured from their joined portions during rolling, resulting in aserious accident.

4) There is proposed a joining method employing a so-called non-contactheating in which the preceding and succeeding metal pieces are heatedwith a space therebetween and the up-setting is then performed forjoining (Japanese Patent Laid-open Publication No. 60-244401). Accordingto this method, although the uniform magnetic flux must be given to themetal pieces over the full width thereof, the uniform magnetic flux isrestrictedly given to the width of not more than 1000 mm mainly from aviewpoint of design in the power supply of the inductor. In the case ofa larger width, for example, a width of 1900 mm, a pair of inductors arerequired (the limit in the capacity of one inductor is 2000 through 3000kW, and practical use of an inductor which has a capacity of not lessthan 4000 kW and can deal with the width of 1900 mm is difficult). It istherefore difficult to give a uniform magnetic flux running through themetal piece over the full width thereof.

5) For heating the preceding and succeeding metal pieces in a relativelyshort time for joining, there is an induction heating rolling method, asa heating means, in which air-core type coil is used to induction-heatthe rear end of the preceding metal piece and the front end of thesucceeding metal piece and the both ends of these metal pieces are thenpressed against each other for joining (Japanese Patent Laid-openPublication No. 60-244401). In this method, however, since the metalpieces to be joined are inserted into the coil and heated, this methodcannot be applied to a metal piece whose dimension is larger than theinner dimension of the coil. On the other hand, in the case where bothmetal pieces have a small width, such a problem does not occur though apart of the magnetic fluxes does not contribute to heat the metalpieces, thereby deteriorating the heating efficiency for the inputpower. Further, in connection with the above item 4), in the case wherethe metal pieces are heated using a plurality of inductors, since themagnetic flux is not generated from a portion corresponding to a partbetween inductors adjacent to each other, the magnetic flux runningthrough the metal piece is locally decreased and the temperature rise atthis portion is insufficient. It is therefore important to make a spacebetween the adjacent inductors as small as possible, but the spacebetween the adjacent inductors cannot be reduced at a stage of securingfacilities. Thus, the temperature distribution of the metal pieces inthe width direction thereof inevitably becomes uneven.

6) Although a preferred temperature at the opposed faces (the end faces)of the metal pieces can be usually within a range of approximately 1350to 1400° C., in the case where the metal pieces joined at such atemperature are rolled by a finishing rolling mill composed of aplurality of stands which functions after joining with a draftpercentage increased by 10 times or more, all kinds of steel cannot berolled without causing rupture at the joining portions until thecompletion of rolling.

In the endless rolling of the metal pieces, since the timing of theprocessing for joining the metal pieces must match with that of therolling process, it is general to provide a movable joining apparatusplaced on the entry side of a group of finishing rolling mills so thatit can follow the movement of the steel pieces, or to provide anapparatus such as a looper having a timing buffer function between thejoining apparatus and the rolling facilities. This involves an extensionof the line or provision of new apparatuses, thereby disadvantageouslyincreasing the facility cost. However, in regard of this problem, aproposition disclosed, e.g., in Japanese Patent Laid-open PublicationNo. 4-89120 has been given and the problem has been already improved.

The present invention intends to achieve the following objects.

1) On the entry side of the hot finishing rolling facilities, thepreceding metal piece and the succeeding metal piece are uniformlyheated over the full width thereof and, when pressing the metal piecesagainst each other, the joining is carried out until a sufficientstrength can be obtained (until such a strength that the plates are notruptured during rolling can be secured), thereby performing a stablerolling operation.

2) The both metal pieces are heated in an appropriate range oftemperature and joined with a sufficient strength being given theretoirrespective of temperature or thickness of the metal pieces to bejoined or kinds of the plates.

3) Even if a plurality of inductors for generating magnetic fluxes areused, a stable heating and joining are realized by making uniform phasesof currents to be supplied to the respective inductors.

4) The metal pieces are heated and joined with a good efficiency in ashort time irrespective of difference in sizes of the metal pieces andwidths of the same in particular.

5) Irrespective of the kinds of the metal pieces to be joined, anexcellent joining is realized by giving such an advantageous heatingcondition that problems such as rupture of the plates do not occur inthe succeeding finishing rolling process.

DISCLOSURE OF THE INVENTION

The above-mentioned objects which are tasks to be solved by theinvention can be attained by adopting the following measures.

1) A method for joining metal pieces wherein a rear end of a precedingmetal piece and a front end of a succeeding metal piece are heated andthe metal pieces are pressed against each other for joining before hotfinishing rolling, characterized in that the rear end of the precedingmetal piece and the front end of the succeeding metal piece are opposedwith a space therebetween, and an alternating magnetic field runningthrough the metal pieces in the thickness direction thereof is generatedat end regions of the opposed faces of the respective metal pieces toperform heating; and another alternating magnetic field reverse withrespect to the former alternating magnetic field is partially generatedin the end regions of the opposed faces of the metal pieces and ineither a region where the metal pieces exist or a region outside widthends of the metal pieces (the outside of the width ends of the metalpieces), thereby adjusting a temperature distribution in the end regionsof the opposed faces of the metal pieces in the width direction thereof.

2) A method for joining metal pieces wherein a rear end of a precedingmetal piece and a front end of a succeeding metal piece are heated andthe metal pieces are pressed against each other for joining before hotfinishing rolling, characterized in that the rear end of the precedingmetal piece and the front end of the succeeding metal piece are opposedwith a space therebetween and an alternating magnetic field runningthrough the metal pieces in the thickness direction thereof is generatedat end regions of the opposed faces of the respective metal pieces toperform heating, wherein another alternating magnetic field reverse withrespect to the former alternating magnetic field is partially generatedin either a region where the metal pieces exist or a region outsidewidth ends of the metal pieces, by arranging reverse magnetic fieldgeneration portions of a plurality of reverse magnetic field generationcircuits each having the reverse magnetic field generating portion and aswitch connected to the reverse magnetic field generating portion in theend regions of the opposed faces of the metal pieces in the widthdirection thereof and controlling so as to open and/or close the switchof each reverse magnetic field generation portion, thereby adjusting atemperature distribution in the end regions of the opposed faces of themetal pieces in the width direction thereof.

3) The region for generating the reverse alternating magnetic fieldpreferably comprises at least one of a region in which a temperaturerapidly reaches a target heating temperature in the width direction ofthe metal pieces as compared with other regions, and a region outside acentral region of the metal piece in the width direction thereof or thewidth ends of the metal piece.

4) A method for joining metal pieces wherein a rear end of a precedingmetal piece and a front end of a succeeding metal piece are heated andthe metal pieces are pressed against each other for joining before hotfinishing rolling, characterized in that the rear end of the precedingmetal piece and the front end of the succeeding metal piece are opposedwith an interval of space therebetween, and an alternating magneticfield running through the metal pieces in the thickness directionthereof is generated at end regions of the opposed faces of therespective metal pieces to perform heating, wherein conductive membersare provided to the both width ends of at least one of the rear end ofthe preceding metal piece and the front end of the succeeding metalpiece with a space between the conductive member and the metal piece,thereby improving the heating efficiency in the width ends of the metalpiece by this member.

5) The members set forth in 4) are preferably arranged over both thepreceding metal piece and the succeeding metal piece.

6) A method for joining metal pieces wherein a rear end of a precedingmetal piece and a front end of a succeeding metal piece are heated andthe metal pieces are pressed against each other for joining before hotfinishing rolling, characterized in that the rear end of the precedingmetal piece and the front end of the succeeding metal piece are opposedwith a space therebetween, and an alternating magnetic field runningthrough the metal pieces in the thickness direction thereof is generatedat end regions of the opposed faces of the respective metal pieces toperform heating, wherein the both width ends of at least one of the rearend of the preceding metal piece and the front end of the succeedingmetal piece are brought into contact with conductive members, therebyimproving the heating efficiency in the width ends of the metal piece bythese members.

7) A method for joining metal pieces wherein a rear end of a precedingmetal piece and a front end of a succeeding metal piece are heated andthe metal pieces are pressed against each other for joining before hotfinishing rolling, characterized in that the rear end of the precedingmetal piece and the front end of the succeeding metal piece are opposedwith a space therebetween, and an alternating magnetic field runningthrough the metal pieces in the thickness direction thereof is generatedin an end region on the opposed faces of the respective metal pieces byan inductor to perform heating, wherein members consisting of magneticsubstance whose depth is two to 10 times as large as an osmotic depth d₀of induced current which can be expressed by the following equation areprovided in a gap between each of the metal piece and the inductor andin a region which is not more than 10 times as large as the osmoticdepth d₀ and inside the width end of the metal piece, thereby enhancingthe density of the magnetic flux of the alternating magnetic field toimprove the heating efficiency in the width ends of the metal piece bythese members.

d ₀={ρ×10⁷/(μ×f)}^(½)/2π, where

d₀: osmotic depth of the induced current (m)

f: frequency of the alternating magnetic field (Hz)

ρ: electric resistivity (Ω*m)

μ: relative magnetic permeability

8) A method for joining metal pieces wherein a rear end of a precedingmetal piece and a front end of a succeeding metal piece are heated andthe metal pieces are pressed against each other for joining before hotfinishing rolling, characterized in that the rear end of the precedingmetal piece and the front end of the succeeding metal piece are opposedwith a space therebetween, and an alternating magnetic field runningthrough the metal pieces in the thickness direction thereof is generatedin an end region on the opposed faces of the respective metal pieces byan inductor to perform heating, wherein an overlap width L (m) of eachmetal piece and a magnetic pole of the inductor in the longitudinaldirection of each metal piece is so adjusted as to satisfy the followingexpression.

L≧2*d ₀, where

d₀: osmotic depth of the induced current (m)

(d ₀={ρ×10⁷/(μ×f)}^(½)/2π) where

f: frequency of the alternating magnetic field (Hz)

ρ: electric resistivity (Ω*m)

μ: relative magnetic permeability

9) A method for joining metal pieces wherein a rear end of a precedingmetal piece and a front end of a succeeding metal piece are heated andthe metal pieces are pressed against each other for joining before hotfinishing rolling, characterized in that the rear end of the precedingmetal piece and the front end of the succeeding metal piece are opposedwith a space therebetween, and an alternating magnetic field runningthrough the metal pieces in the thickness direction thereof is generatedin an end region on the opposed faces of the respective metal pieces byan inductor to perform heating, wherein a relative position of amagnetic pole of the inductor and each metal piece in the longitudinaldirection is changed to adjust a penetration quantity of the magneticfluxes of the alternating magnetic field for each magnetic piece.

10) A method for joining metal pieces wherein a rear end of a precedingmetal piece and a front end of a succeeding metal piece are heated andthe metal pieces are pressed against each other for joining before hotfinishing rolling, characterized in that the rear end of the precedingmetal piece and the front end of the succeeding metal piece are opposedwith a space therebetween, and an alternating magnetic field runningthrough the metal pieces in the thickness direction thereof is generatedin an end region on the opposed faces of the respective metal pieces bya plurality of inductors arranged in the width direction of the metalpieces to heat the respective metal pieces, wherein a synchronouscontrol of the phase is performed in such a manner that the currenthaving the same phase flows in a coil of each inductor.

11) A method for joining metal pieces wherein a rear end of a precedingmetal piece and a front end of a succeeding metal piece are heated andthe metal pieces are pressed against each other for joining before hotfinishing rolling, characterized in that the rear end of the precedingmetal piece and the front end of the succeeding metal piece are opposedwith a space therebetween, and an alternating magnetic field runningthrough the metal pieces in the thickness direction thereof is generatedin an end region on the opposed faces of the respective metal pieces bya plurality of inductors arranged in the width direction of the metalpieces to heat the respective metal pieces, wherein the space betweenmagnetic poles of the inductors adjacent to each other is set to notmore than five times as large as an osmotic depth d₀ of the inducedcurrent represented by the following expression.

d ₀={ρ×10⁷/(μ×f)}^(½)/2π, where

d₀: osmotic depth of the induced current (m)

f: frequency of the alternating magnetic field (Hz)

ρ: electric resistivity (Ω*m)

μ: relative magnetic permeability

12) A method for joining metal pieces wherein a rear end of a precedingmetal piece and a front end of a succeeding metal piece are heated andthe metal pieces are pressed against each other for joining before hotfinishing rolling, characterized in that a temperature T (° C.) in aheating region of the preceding metal piece and the succeeding metalpiece is adjusted to be within a range of the following expression.

T _(S) ≦T≦(T _(S) +T _(L))/2, where

T_(S): solidus line temperature of the metal piece (° C.)

T_(L): liquidus line temperature of the metal piece (° C.)

13) A method for joining metal pieces wherein a rear end of a precedingmetal piece and a front end of a succeeding metal piece are heated andthe metal pieces are pressed against each other for joining before hotfinishing rolling, characterized in that a temperature T (° C.) in aheating region of the preceding metal piece and the succeeding metalpiece is adjusted to be within a range of the following expression.

if T_(c)≦T_(S),

(T _(c) +T _(S))/2≦T≦(T _(S) +T _(L))/2, and

if T_(c>T) _(S)

T _(S) ≦T≦(T _(S) +T _(L))/2

where T_(S): solidus line temperature of the metal piece (° C.)

T_(L): liquidus line temperature of the metal piece (° C.)

T_(c): melt temperature of an iron oxide scale (° C.)

14) An apparatus for joining metal pieces comprising an inductor havingat least a pair of magnetic poles sandwiching the metal pieces in thethickness direction thereof with a gap therebetween, characterized inthat a reverse magnetic field generation portion of a circuit forgenerating an alternating magnetic field whose direction is reversedfrom that of an alternating magnetic field generated by the inductor isprovided between the magnetic poles of the inductor.

15) An apparatus for joining metal pieces comprising: an inductor havingat least a pair of magnetic poles sandwiching the metal pieces in thethickness direction with a gap therebetween; and a clamp for verticallyclamping a preceding metal piece and a succeeding metal piece betweenthe magnetic poles of the inductor for fixing and holding the metalpieces, characterized in that the clamp protrudes from a region wherethe metal pieces are fixed and held toward the ends of the metal piecesand has a plurality of notch portions made by notching the clamp in thecomb-like form at an interval of space in the width direction of themetal pieces, and a reverse magnetic field generation portion of areverse magnetic field generation circuit for generating an alternatingmagnetic field whose direction is reversed from that of an alternatingmagnetic field generated by the inductor is provided to each of thenotch portions.

16) A plurality of reverse magnetic generation portions are preferablyprovided along the width direction of the inductor. Further, each of thereverse magnetic generation portions is preferably composed of a coil ofone wind or of a plurality of winds or a U-shaped conductive member andfurther has a member consisting of magnetic substance.

17) The reverse magnetic filed generation circuit has a switch foropening/closing the circuit and preferably has a variable resistor.

18) An apparatus for joining metal pieces comprising an inductor havingat least a pair of magnetic poles sandwiching the metal pieces in thethickness direction thereof with a gap therebetween, characterized bycomprising a reverse magnetic field generation circuit having: a reversemagnetic field generation portion for generating an alternating magneticfield whose direction is reversed from that of an alternating magneticfield generated by the inductor; a switch; a lead wire for connectingthe switch and the reverse magnetic field generation portion to eachother; and an open/close controller for opening and/or closing theswitch.

19) An apparatus for joining metal pieces comprising an inductor havingat least a pair of magnetic poles sandwiching the metal pieces in thethickness direction thereof with a gap therebetween, characterized bycomprising a member consisting of magnetic substance which is providedbetween the magnetic poles and increases a density of magnetic flux ofan alternating magnetic field generated by the inductor at width ends ofthe metal pieces.

20) An apparatus for joining metal pieces comprising an inductor havingat least a pair of magnetic poles sandwiching the metal pieces in thethickness direction thereof with a gap therebetween, characterized bycomprising a conductive member which is provided between the magneticpoles and outside width ends of the metal pieces in the width directionwith a gap or provided contacting with the width ends.

21) An apparatus for joining metal pieces comprising an inductor havingat least a pair of magnetic poles sandwiching the metal pieces in thethickness direction thereof with a gap therebetween, characterized bycomprising a moving mechanism being capable of moving the inductor inthe longitudinal direction of the metal pieces.

22) An apparatus for joining metal pieces comprising at least two setsof inductors each having a pair of magnetic poles sandwiching the metalpieces in the thickness direction thereof, characterized in that a powersupply inverter is provided to each of the inductors and each powersupply inverter is connected to a phase control circuit.

23) An apparatus for joining metal pieces comprising at least two setsof inductors each having a pair of magnetic poles sandwiching the metalpieces in the thickness direction thereof, characterized by comprisingprojections provided on adjacent surfaces of the magnetic poles of theinductors in such a manner that the projections are brought into contactwith each other or decrease an interval of space therebetween.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory view showing the point of heating according tothe present invention;

FIG. 2 is an explanatory view showing the point of heating according toa prior art;

FIG. 3 is an explanatory view showing the state of heating metal pieces;

FIG. 4 is an explanatory view showing the state where cracks aregenerated in a joined portion of metal pieces;

FIG. 5 is a view typically showing a structure of a circuit;

FIG. 6 is a view showing a structure of an apparatus preferable forcarrying out the present invention;

FIG. 7 is a view showing the state of joining metal pieces;

FIG. 8 is an explanatory view showing the state where joined metalpieces are ruptured;

FIG. 9 is a view showing a structure of an apparatus according to thepresent invention;

FIG. 10 is a view showing a primary part of the apparatus according tothe present invention;

FIG. 11 is an explanatory view showing the point of joining metal piecesaccording to the present invention;

FIG. 12 is an explanatory view showing the point of joining metal piecesaccording to the present invention;

FIG. 13 is a view showing an example of a conductive members;

FIG. 14 is a view showing an example of a member consisting of magneticsubstance;

FIG. 15 is a view showing an example of a member consisting of magneticsubstance;

FIG. 16 is a view showing an example of a member consisting of magneticsubstance;

FIG. 17 is a view showing an example of a member consisting of magneticsubstance;

FIG. 18 is a view showing a reverse magnetic field generation portion ofa circuit;

FIG. 19 is a view showing a reverse magnetic field generation portion ofa circuit;

FIG. 20 is a view showing a structure of an equipment for performingcontinuous hot rolling to metal pieces;

FIG. 21 is a view showing a structure of a primary part of a joiningapparatus according to the present invention;

FIG. 22 is an explanatory view showing the state where metal pieces areheated;

FIG. 23 is a view showing a structure of a primary part of a joiningapparatus according to the present invention;

FIG. 24 is an explanatory view showing a case where metal pieces havingdifferent widths are joined;

FIGS. 25(a), (b) and (c) are explanatory views showing cases where metalpieces are heated and joined in accordance with the present invention;

FIGS. 26(a), (b) and (c) are explanatory views showing cases where metalpieces are heated and joined in accordance with the present invention;

FIG. 27 is an explanatory view showing the point of joining metal piecesin accordance with the present invention;

FIG. 28 is a view showing an example in which metal pieces are heatedusing a single inductor;

FIG. 29 is a view showing an example in which metal pieces are heatedusing a single inductor;

FIG. 30 is a view showing a distribution of a temperature rising speedin end portion of a metal piece in the width direction thereof;

FIGS. 31(a) and (b) are views showing an example in which metal piecesare heated and joined by arranging members consisting of magneticsubstance;

FIG. 32 is a view showing an example in which metal pieces are heatedand joined by arranging members consisting of magnetic substance;

FIG. 33 is a view showing the relationship between the width dimensionof a member consisting of magnetic substance/osmotic depth of an inducedcurrent, and the length of joining failure;

FIG. 34 is a view showing a structure of a primary part of a joiningapparatus to which members consisting of magnetic substance arearranged;

FIG. 35 is an explanatory view showing the point of movement of membersconsisting of magnetic substance;

FIGS. 36(a), (b) and (c) are views showing a structural example of aninductor;

FIGS. 37(a), (b) and (c) are views showing a structural example of aninductor;

FIGS. 38(a) and (b) are views showing a structural example of aninductor;

FIG. 39 is a view showing the state of distribution of magnetic fluxes;

FIG. 40 is a view showing the state of distribution of magnetic fluxes;

FIG. 41 is a graph showing the relationship between L/d₀ and thetemperature rising speed;

FIG. 42 is an explanatory view showing the state in which metal piecesare heated;

FIG. 43 is an explanatory view showing a case where a penetrationquantity of magnetic fluxes is adjusted;

FIG. 44 is an explanatory view showing a case where a penetrationquantity of magnetic fluxes is adjusted;

FIG. 45 is view showing the relationship between the temperature risingspeed ratio and L/d₀ (space g);

FIG. 46 is a view showing a structure of a moving mechanism of aninductor;

FIG. 47 is a view showing a structure of a moving mechanism of aninductor;

FIG. 48 is a view showing a structure of a moving mechanism of aninductor;

FIGS. 49(a) and (b) are explanatory views showing the points of heatingand joining metal pieces;

FIG. 50 is a view showing the state in which an inductor is arranged;

FIGS. 51(a) and (b) are views showing structures in a case where aplurality of inductors are used;

FIG. 52 is a block diagram showing a control system of inductorsaccording to the present invention;

FIG. 53 is a view showing a structure in a case where a plurality ofinductors are used;

FIG. 54 is a view showing a structure of a joining apparatus for metalpieces;

FIG. 55 is a view showing the state in which metal pieces are heated;

FIG. 56 is view showing the state in which inductors are arranged;

FIG. 57 is a graph showing the relationships between W₁/d₀ and such alength that a temperature rising speed is not more than 90%, and betweenW₁/d₀ and a temperature rising speed ratio;

FIG. 58 is a view showing, especially, inductors of an apparatusaccording to the present invention;

FIG. 59 is a graph showing the relationship between the C content andthe heating temperature;

FIG. 60 is a graph showing the relationship between the C content andthe heating temperature;

FIG. 61 is a view showing sections of ends of metal pieces cut by a cropshear;

FIG. 62 is a view showing a structure of an electric circuit;

FIG. 63 is a graph showing the relationship between the plate width andthe temperature at a joining face;

FIG. 64 is a graph showing the relationship between the plate width andthe temperature rising speed ratio;

FIG. 65 is a graph showing the relationship between the distance from anend of a plate and the temperature rising speed ratio;

FIG. 66 is a graph showing the relationship between the distance in awidth direction of a metal piece and the temperature at a joining face;

FIG. 67 is a graph showing the relationship between the distance in adirection of a metal piece width and the temperature of a joining face;

FIG. 68 is graph showing the relationship between the distance in adirection of a metal piece width and the calorific power ratio at ajoining face;

FIG. 69 is a view showing a state in which an inductor is arranged;

FIG. 70 is a view typically showing inductors;

FIG. 71 is a graph showing the relationship between the width dimensionof a metal piece and the rate of heating quantity;

FIG. 72 is a graph showing the relationship between the length in awidth direction of a metal piece and the rate of heating quantity; and

FIG. 73 is a view showing an example of an inductor which is preferablefor embodying the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

In case of heating metal pieces to be joined by generating analternating magnetic field running through the metal pieces in athickness direction thereof, the present invention partially generatesanother alternating magnetic field reversed from the above alternatingmagnetic field in either of a region where the metal pieces exist oranother region outside the width ends of the metal pieces to adjust thetemperature distribution in the plate width direction. The presentinvention will now be described in detail hereinafter with reference tothe drawings.

FIG. 1 shows a structure of a joining apparatus preferable for adjustingthe temperature distribution at ends of metal pieces in the widthdirection thereof in case of heating and joining the metal pieces. Inthe drawing, reference numeral 1 denotes a metal piece (referred to as apreceding metal piece hereinbelow) precedently carried; 2, a metal piece(referred to as a succeeding metal piece hereinbelow) carried after thepreceding metal piece 1; 3, at least a pair of inductors for heatingcomposed of a coil c and a core t and sandwiching the preceding metalpiece 1 and the succeeding metal piece 2 in the thickness directionthereof with a gap D (although the gap D is a space, any electricalinsulator may be provided thereto) therebetween. An alternating magneticfield running through the metal pieces in the thickness directionthereof is generated by the inductors 3 to heat the end region of theopposed faces until the temperature in this region reaches apredetermined value.

Further, reference numeral 4 designates a power supply of the inductors3; and 5, a reverse magnetic field generation portion of a reversemagnetic field generation circuit (an electric circuit). A lead wire 5 ais connected to the both ends of the loop type reverse magnetic fieldgeneration portion 5 to form a closed circuit including these ends, andan alternating magnetic field reversed from the alternating magneticfield produced by the inductors 3 is generated by flowing an inducedcurrent when heating by the inductors 3 or by positively flowing acurrent (from any other power supply, for example). In addition,reference numeral 6 represents a switch having an open/close controllerr; and 7, a variable resistor.

As shown in FIG. 2, the preceding metal piece 1 and the succeeding metalpiece 2 are opposedly provided with a gap g therebetween, which is a gapof approximately a few to tens mm and may be an interval of space, orany other electrical insulator may be provided to this gap. When thealternating magnetic field running in the plate thickness direction isgenerated by the inductors 3, a current e as shown in FIG. 3 is inducedby the alternating magnetic field in the end region of the opposed facesof the metal pieces which will be joined to each other, and this portionis heated in a extremely short time by the heat produced at this time.

Since the current e is difficult to flow at corners f of the metalpieces 1 and 2, the degree of temperature rise is small at the cornersf. If the heating and temperature rise are tried until the temperatureat which the joining is possible, the central region in the plate widthdirection is likely to be melted down. On the other hand, even if thejoining of the metal pieces only in the central region thereof is tried,cracks such as shown in FIG. 4 may be developed during rolling becauseof insufficiency of the joining strength in the width end region, andthere is such a disadvantage that the rolling cannot be continued.

In the present invention, since portions to be joined are heated by theinductors 3 and the alternating magnetic field reversed from thealternating magnetic field d produced by the inductors 3 is generated bysuch a reverse magnetic field generation circuit as shown in FIG. 5 (byflowing the induced current in the circuit by main magnetic fluxes ofthe inductors 3 or positively flowing such a current that the reversemagnetic field is generated) in a region where the temperature largelyvaries (a region which is located in the width direction of the metalpieces and where the temperature reaches a target heating temperaturefaster than in any other regions) and in a central region in the platewidth direction where the temperature greatly raises in particular toweaken the strength of the magnetic field in the portions to be joined,an excessive heating in these portions can be suppressed.

Although the heating time is prolonged to some extent because theheating is carried out with such a reverse magnetic field beinggenerated, the corners of the metal pieces at which the temperature riseis difficult can be heated until the temperature reaches a predeterminedrange without a fear of melt down in the central region in the platewidth direction. As a result, the temperature distribution in the widthdirection becomes substantially uniform, thereby ensuring thesatisfactory joining strength.

In a concrete structure of an apparatus for adjusting the temperature,as shown in FIG. 6, a plurality of reverse magnetic field generationportions 5 are previously arranged in parallel with the plate widthdirection of the metal pieces 1 and 2, and switches 6 to be connected tothe reverse magnetic field generation portions 5 which are located in aregion where suppression of the temperature rise is desired when heatingare closed to generate a reverse magnetic field.

Further, if the quantity of current flowing in the reverse magneticfield generation circuit is varied by adjusting a variable resistor 7 asshown in FIG. 1, for example, the scale of the reverse magnetic field isadjusted, thereby further accurately adjusting the temperature. In thecase where a plurality of reverse magnetic field generation circuits areprovided, it is effective to provide a variable resistor 7 for eachcircuit for adjusting an impedance of each circuit. It is also possibleto additionally provide a coil, a capacitor or others, and at least oneof these members may be used to adjust the impedance of each reversemagnetic field generation circuit. If a desired impedance is given toeach reverse magnetic field generation circuit by a variable resistor 7,for example, a desired strength of the reverse magnetic field can beobtained. Also, phases of the respective circuits are unified, therebyrelatively freely adjusting the temperature of the metal pieces in thewidth direction thereof.

In the state of arrangement as shown in FIG. 6, in the case where thereverse magnetic field generation portions 5 are arranged outside theplate width ends of the metal pieces and the reverse magnetic generationcircuits are closed to flow the induced current therethrough, magneticfluxes outside the width ends of the magnetic pieces can be converged atthe corners f of the magnetic pieces, and it is hence extremelyadvantageous to enhance the heating efficiency at the corners f.

When heating the metal pieces, the metal pieces may be pressed againsteach other (the pressing may be carried out while heating or afterheating). In such a case, the preceding metal piece 1 and the succeedingmetal piece may be joined being shifted from each other (this state isreferred to as dislocation). When the dislocated portion is caught in aroller, an end of one metal piece is bent toward an end of the othermetal piece. The dislocated portion deeply encroaches upon a soundportion which will be a product as the number of rolling passesincreases as shown in FIG. 8, whereby a thin portion is locally formed.On the other hand, the plate is ruptured due to a variation in a tensileforce between stands during rolling, and the rolling may not becontinued.

In the present invention, therefore, a dislocation preventing plate 8having notches u whose tips are opened along the width direction of themetal pieces 1 and 2 is provided as shown in FIG. 9 to join the metalpieces 1 and 2.

In FIG. 9, by connecting the dislocation preventing plate 8 to eachclamp 9 having a positioning function for the metal pieces 1 and 2,vertical dislocation of the metal pieces 1 and 2 which can be causedduring pressing can be suppressed, so as to minimize the dislocation.

The dislocation preventing plate 8 can avoid the temperature rise ofitself during heating the metal pieces 1 and 2 and has the notches u forsecuring the strength as a pressure plate. When the reverse magneticfield generation portion 5 is engaged with each notch u and the reversemagnetic field is appropriately generated by each reverse magnetic fieldgeneration circuit including the reverse magnetic field generationportion, the entire area of the portions of the metal pieces which willbe joined can be substantially uniformly heated.

FIG. 10 shows a primary part in a state where the reverse magnetic fieldgeneration portions 5 are provided on the dislocation preventing plate8. As shown in FIG. 10, when loops are formed by closing the switches 6provided at positions where the suppression of heating is desired, acurrent e′ flows in each reverse magnetic field generation portion 5 andeach lead wire 5 a in such a direction that the magnetic flux (reversemagnetic flux) which is directed to cancel out the main magnetic flux isgenerated by the action of electromagnetic induction. In this case, themain magnetic flux is weakened by the reverse magnetic flux to suppressthe temperature rise of the metal pieces at portions where the switches6 are closed.

FIG. 11 shows a structural example of such an apparatus that the degreeof temperature rise in the central region of the metal pieces 1 and 2 inthe width direction thereof is decreased and the magnetic flux outsidethe width ends of the metal pieces is converged at the corners f.Further, FIG. 12 shows a structural example of an apparatus which candecrease the degree of temperature rise only in the central region ofthe metal pieces 1 and 2 in the width direction thereof.

As the dislocation preventing plate, SUS304 and others can be adopted,though any other material having a strength at a high temperature suchas titanium, tungsten and others may be used.

In this invention, although the U-shaped conductive member (such as Cu)is exemplified as the reverse magnetic field generation portion 5 of thereverse magnetic field generation circuit, a coil type member such asthat shown in FIG. 13 can be adopted. In this case, the effect isenhanced as the number of winds increases, though the winds are notrestricted to any particular number.

In order to increase a value of the current flowing in each reversemagnetic field generation circuit without additionally providingunnecessary units, in this invention, as shown in FIG. 14 or 15, amember M consisting of magnetic substance (a silicon steel plate orothers) may be provided within each reverse magnetic field generationportion 5.

Further, in order to prevent the member M from being heated, it ispreferable to obtain a structure in which a plurality of members M aresuperimposed one above the other through insulating films as shown inFIGS. 16 and 17.

Note that such a plate type superimposed body (magnetic substance) asshown in FIG. 16 may be provided in each reverse magnetic fieldgeneration circuit having a coil type reverse magnetic field generationportion 5 shown in FIG. 17, instead of a cylindrical superimposed body(magnetic substance).

FIGS. 18 and 19 show examples in which the members M consisting ofmagnetic substance are assembled in the reverse magnetic fieldgeneration portions 5 of the reverse magnetic field generation circuits.

FIG. 20 shows an equipment for performing continuous hot rolling to themetal pieces, and a joining apparatus A preferable for embodying thisinvention is provided, for example, between pinch rolls 11 and 12 on thedelivery side of a cutting apparatus (a shear or others) 10.

In FIG. 20, reference numeral 13 denotes a rewinder for rewinding ametal piece wound in a coil form; 14, a pinch roll; 15, a leveler; 16, ascale breaker; and 17, a group of finishing rolling mills. If thejoining apparatus A is of a fixed type in this equipment, a looper isprovided on the entry side of the scale breaker 16.

Each reverse magnetic field generation circuit constituted by thereverse magnetic field generation portion 5 may be maintained closed(the switch 6 is closed) by the open/close controller r from an initialstage when heating the metal pieces, or may be closed during heating.The usage of the circuit is not restricted to a specific procedure.

Although the above has been described in connection with the case wherethe degree of temperature rise is suppressed especially in the regionwhere the temperature is locally increased when heating the metalpieces, the temperature in this region can be precedently increased bygenerating in the circuit the alternating magnetic field whose directionis same with that of the alternating magnetic field produced by theinductors 3 (in this case, the current is positively supplied to thecircuit).

The description will now be given as to a case where an apparatus havingsuch a structure as shown in FIG. 21 is used to improve the heatingefficiency at the width ends of the metal pieces.

In FIG. 21, reference numeral 18 designates a conductive member whichhas been described as an example of the plate, and each member 18 isprovided between magnetic poles of the inductor 3 and provided outsidethe ends of the metal pieces 1 and 2 in the width direction thereof witha gap t₁ between the members 18 and the metal pieces 1 and 2. In thiscase, the gap t₁ between may be an interval of space, or any electricalinsulator may be provided herein.

The preceding metal piece 1 and the succeeding metal piece 2 are clampedby clamps 19 and 20 with a space g, namely, a small gap being providedbetween the ends of the opposed faces of the metal pieces 1 and 2, andare heated by generating the alternating magnetic field running throughthe metal pieces 1 and 2 the thickness direction thereof by the inductor3 having a pair of magnetic poles vertically sandwiching the metalpieces 1 and 2. At this time, the induced current caused due to thealternating magnetic field also flows in each conductive member 18, andthe temperature is increased at the same speed with that in the centralregion of the pieces 1 and 2 at the corners of the metal pieces wherethe temperature is not likely to be increased.

By providing each conductive member 18 in close proximity to one side ofthe metal pieces, the entire end region of the opposed faces to bejoined can be heated at the same speed (a uniform heating over the fullwidth). The reason thereof is as follows.

In the first place, in the conventional heating and joining method inwhich the conductive members 18 are not provided, the quantity ofmagnetic fluxes is small at ends of the metal pieces, which will bejoined, in the width direction thereof, so that the induced current eflows in the circular form as shown in FIG. 22. Even if the skin effectof the induced current flowing in each end of the metal pieces 1 and 2can be expected, the induced current is difficult to flow through theend of each metal piece in the width direction thereof, whereby thetemperature rise is insufficient. As a result, even though the rear endof the preceding metal piece and the front end of the succeeding metalpiece are opposedly joined to each other, the joining strength is low atthe ends of the joined portions in the width direction of the steelpieces. In the rolling operation to be subsequently performed, cracksproduced at the ends propagate to the central portion in the widthdirection, resulting in the rupture.

In the case where the conductive members 18 are provided with a space t₁between the both width ends of at least one of the preceding metal piece1 and the succeeding metal piece 2 and the conductive members 18 inaccordance with the invention as shown in FIG. 21, however, thealternating magnetic field produced between the magnetic poles of theinductor 3 also runs through the conductive members 18, therebygenerating the induced current e in each member 18. Since this inducedcurrent and the induced current generated in the metal pieces 1 and 2flow in opposed directions in the region where these currents are inclose proximity to each other, they are attracted to each other. Theinduced current produced in the metal pieces consequently flows to becloser to the ends of the metal pieces in the width direction thereof.The temperature rising speed at the ends of the metal pieces in thewidth direction thereof becomes closer to the temperature rising speedin the central region of the same in the width direction thereof, andhence the temperature can be substantially uniformly increased to avalue in the entire region of the portions to be joined in the widthdirection. Further, by securing the sufficient pressing force whenpressing the metal pieces against each other, the complete joining canbe carried out over the full width of the metal pieces including theends of the metal pieces in the width direction thereof.

As for the conductive member suitable for the invention, the desiredobject can be attained only if any member can generate the inducedcurrent having a predetermined scale, the member is not thus restrictedto a specific type.

However, a copper plate produces less heat due to the induced currentand is inexpensive, and hence it is preferable. Further, since amaterial having a high melting point such as a tungsten plate or agraphite plate has a durability against heat, such a material can alsobe used. Furthermore, a steel plate or an Al plate can be used for along time if a cooling means is additionally provided.

Although FIG. 21 shows an example in which each conductive member 18 isprovided over the preceding metal piece 1 and the succeeding metal piece2, the invention is not restricted to the example in this figure. Thesame effect can be obtained when separate conductive members 18 a and 18b are provided to the rear end of the preceding metal piece and thefront end of the succeeding metal piece, respectively. In this case, itis particularly advantageous when the metal pieces having differentwidths are joined.

In regard of the width dimension of the conductive member, when thedimension is too small, generation of the induced current is difficulteven though the magnetic flux runs through the member from the inductor,thereby requiring such a width that the induced current can be produced.The width of the conductive member can be appropriately changed only ifthis condition is satisfied. The member is not restricted to havespecific thickness and length.

The space t₁ between each conductive member and the metal piece must beprovided so that the induced current produced in the metal piece and theinduced current generated in the conductive member are attracted to eachother and the conductive member and the metal piece are in closeproximity to each other. Specifically, the space t₁ may be over 10 mm,though it may preferably be set to 3 through 5 mm.

FIG. 24 illustrates an example in which the metal pieces havingdifferent width dimensions are joined with the centers of the metalpieces in the width directions thereof being matched. When thealternating magnetic field generated by the inductor 3 runs through eachconductive member 18, the induced current is produced in the member 18.The induced currents produced in the metal pieces 1 and 2, therefore,flow in the vicinity of the ends of the metal pieces in the widthdirection thereof by the interaction of the induced current in themember 18 and the induced currents generated in the metal pieces,enabling a uniform heating over the entire area in the plate widthdirection.

Although the desired object can be attained by providing the conductivemembers 18 only at the both width ends in the end portion of the narrowmetal pieces as shown in FIG. 24, the effect can be further enhanced byadditionally providing the conductive members at the both width ends ofthe wide metal piece as indicated by a two-dotted line in FIG. 24.

As apparent from the above-mentioned action and effect of the conductivemember 18, in the invention, the magnetic field generated by theinductor having at least a pair of magnetic poles must run through theconductive member 18. That is, the conductive member 18 must be providedin such a region that the magnetic flux produced between the magneticpoles of the inductor 3 runs therethrough.

As viewed from the relationship between the width of each metal pieceand the width of the inductor, in this invention, it is preferable toprovide the inductor in such a manner that the inductor overlaps on themetal pieces and the magnetic flux generated by the inductor runsthrough the metal piece and at least a part of the conductive member.Specifically, it is preferable that the inductor (core) having a widthdimension larger than that of the metal pieces to be joined is used soas to protrude from the metal pieces at the width ends thereof, and theopposed magnetic poles of the inductor face to the metal pieces and theconductive members provided to the width ends of the metal pieces. Thismeans that the metal pieces are provided inside the ends in the widthdirection at which the quantity of magnetic fluxes is apt to bedecreased, and this arrangement is extremely effective for uniformlyheating the metal pieces in the width direction thereof.

In the case where the width direction of the metal pieces used forjoining is larger than that of the inductor, a plurality of inductorsare preferably aligned in the width direction of the metal pieces so asto protrude from the metal pieces at their width ends.

FIGS. 25(a), (b) and (c) show an example in which a current whose phaseis substantially same with that of the induced current generated in themetal pieces positively flows in the conductive members from outside.

FIG. 25(a) is a top plan view showing a region in which the rear end ofthe preceding metal piece and the front end of the succeeding metalpiece are opposed to each other; FIG. 25(b), a sectional view takenalong the A—A line in FIG. 25(a); and FIG. 25(c), a sectional view takenalong the B—B line in FIG. 25(a).

The inductor 3 shown in FIG. 25 is obtained by winding coils c around acore t constituting a pair of magnetic poles sandwiching the metalpieces and has a width larger than those of the preceding metal piece 1and the succeeding metal piece 2. According to the inductor having sucha structure, since the entire area of the metal pieces in the widthdirection are positioned between the magnetic poles, the magnetic fieldgenerated by the inductor 3 can effectively act on the metal pieces.Note that such an inductor is also disclosed in Japanese PatentLaid-open Publication No. 4-89109.

In FIG. 25, when the (alternating) current whose phase is the same with,but whose direction is reversed from, those of the induced current issupplied from an external power supply 22 to the conductive members 18,the induced current circulating in the metal pieces flows to the widthends thereof, enabling heating the entire region in the width direction.

As similar to FIG. 25, FIGS. 26(a), (b) and (c) show an example in whichthe current is supplied from outside to the conductive members 18 touniformly heat the metal pieces in the full width direction.

FIG. 26 shows the case where the metal pieces each having a width largerthan that of the inductor 3 are heated. In the example illustratedherein, a pair of inductors 3 shown in FIG. 25 are prepared and arrangedalong the width direction of the metal pieces to carry out heating.

FIG. 26(a) is a top plan view showing an apparatus; FIG. 26(b), asectional view taken along the A—A line in FIG. 26(a); and FIG. 26(c), asectional view taken along the B—B line in FIG. 26(a).

In the example shown in FIG. 26, as similar to FIG. 25, the inductors 3each having a substantially C-shaped core t are provided at the ends ofthe metal pieces to be joined, and the conductive members 18 areprovided at the both ends of the metal pieces at an interval of space.Also, the current whose phase is same with that of the induced currentgenerated in the metal pieces is supplied from the external power supply22 to the members 18.

In FIGS. 25 and 26, although the description has been given to the casewhere the inductor having a larger width dimension than those of themetal pieces to be joined is used or the case where at least a pair ofinductors are used to heat the metal pieces, the present invention isnot restricted to the illustrated examples.

Since the scale of the current supplied from the external power supplyto the conductive members is not limited, it is possible to flow thecurrent which is sufficiently large for uniformly heating the ends ofthe metal pieces in the full width direction, and there is no problem ifan inductor whose width is smaller than those of the metal pieces isused.

In the present invention, the metal pieces are heated and joined inaccordance with the above-mentioned points. As a combination of thesepoints, there is adopted a method by which the rear end of the precedingmetal piece and the front end of the succeeding metal piece arepositioned in close proximity with a space (small gap) therebetween and,after heating is performed until the temperature reaches a target value,the power input to the inductor is stopped to carry out the pressing, ora method by which the input power is lowered to such a level that nospark is produced if the temperature at the portions to be joinedreaches the target value, and pressing is started while continuingheating.

The description will now be given as to the case where the conductivemembers are brought into contact with the both width ends of either therear end of the preceding metal piece or the front end of the succeedingmetal piece and the heating efficiency at the width ends of the metalpieces is improved by the members.

Referring to FIG. 27, in case of heating the preceding steel piece 1 andthe succeeding steel piece 2, when the conductive members 23 arepreviously pressed against the both width ends of at least one of themetal pieces in the vicinity of the end thereof, the induced currentgenerated in the preceding metal piece 1 and the succeeding metal piece2 also flow in the members 23. The temperature at the both width ends,i.e., the corners of the metal pieces increases at the same speed withthat in any other regions by the Joule heat generated at this time, andthe problems that the joining failure occurs or the strength in thejoining portions is insufficient due to the resistance at the heatdefective portion are eliminated.

Since each conductive member 23 shown in FIG. 27 is likely to be meltedand fused during heating the metal pieces if its melting point is samewith or lower than that of the metal pieces, it is preferable to use amaterial having a higher melting point than that of the metal pieces,for example, tungsten or carbon.

Further, in regard of a size of each conductive member, the thickness ofthe member is preferably equal to that of the metal pieces to be joined,and it is preferable to adapt such a width that the temperature rise atthe ends of the metal pieces in the width direction thereof is notinsufficient when heating is carried out without the conductive members.Further, the length of the member is preferably not less than a lappinglength as viewed from the top plan of the core of the inductor and themetal pieces to be heated.

In case of heating the metal pieces, the spark is likely to be generatedbetween the conductive members and the metal pieces, Thus, it isparticularly preferable to increase the bearing to or above 2 kg/mm² andpush the conductive members against the metal pieces in advance.

Although FIG. 27 shows the example in which the metal pieces are heatedby using a pair of inductors, it is needless to say that the same effectcan be obtained by using a single inductor such as that shown in FIG. 28to perform heating.

FIG. 30 shows for reference a distribution of the temperature risingspeed (which is measured at a point spaced apart from the end face by1.5 mm and positioned in the center in the plate thickness direction onthe metal piece) in the plate width direction in the case where twometal pieces each consisting of stainless steel of SUS304 and having athickness of 30 mm are heated by using the inductor having a core whosesize is 240×1000 mm as shown in FIG. 29 under such conditions thatdistances between the metal pieces and the core is 90 mm on the upperside and 90 mm on the lower side and the input power is 980 kW. Thedegree of the temperature rise is particularly small at the corners ofthe metal pieces as compared with any other regions. Therefore, evenwhen the metal pieces are subjected to continuous rolling after beingjoined by the inductor having such a heating characteristic, cracks aredeveloped in the joined portion as the rolling proceeds, and it isobvious that the joined portion is inevitably ruptured over the fullwidth.

The description will now be given as a case where, when heating themetal pieces, the density of magnetic fluxes of the alternating magneticfield is increased and the heating efficiency at the width ends of themetal pieces is improved by members each consisting of magneticsubstance provided between each metal piece and the inductor touniformly heat the entire region in the width direction with referenceto FIGS. 31(a) and (b) and FIG. 32.

In case of heating the metal pieces by the inductor 3, members 24consisting of magnetic substance are provided at corners where thedegree of temperature rise is small and the magnetic influx density atthe corners is increased by the members 24. The induced current thenflows closer to these portions, and the degree of heating is alsoincreased. A satisfactory joining strength can thus be secured at theboth width ends without a fear that the metal pieces are melted down inthe central region thereof in the plate width direction.

In this invention, each of the members 24 consisting of magneticsubstance must be provided at a position in a region which is extendingfrom the width end of each metal piece and whose length is not more thanten times as long as the osmotic depth d₀ of the induced current. Thatis because the temperature rise is insufficient in the region and, ifeach member 24 consisting of magnetic substance is inwardly providedbeyond this region, the degree of temperature rise of the metal piecebecomes too large in such a region where the member 24 is provided,causing such a problem as melt down.

A width dimension W of the member 24 is set to be two to ten times aslong as the osmotic depth d₀. That is because the effect obtained byplacing the magnetic substance is reduced when the range in which thedegree of heating and temperature rise can be increased is small, if thewidth dimension W is not more than two times as long as the osmoticdepth d₀. On the other hand, if it is more than 10 times as long as theosmotic depth d₀, the temperature rising speed is extremely increased inthe portion extending beyond the depth d₀, thereby causing melt down. Inorder to perform heating in such a manner that the temperature risingspeed similar to that in the central region of the metal pieces in thewidth direction thereof can be obtained and the temperature distributionis substantially uniform in the entire area in the width direction, thewidth W of each member 24 consisting of magnetic substance is restrictedto a width which is two to ten times as long as the osmotic depth d₀.

FIG. 33 shows the relationship between the width dimension of the member24 consisting of magnetic substance/the osmotic depth d₀ and a length ofdefective joining.

As an example of a continuous hot rolling equipment to which the joiningapparatus having the structure shown in FIG. 31 is incorporated, it ispossible to adopt one shown in FIG. 20 by which the metal pieces arejoined while matching with a timing for rewinding a coil type metalpiece provided between the pinch rolls and a timing for effecting therolling process.

A dimension L_(m) of the member 24 consisting of magnetic substancealong the longitudinal direction of the metal pieces may preferably be2d₀+g+α(α is a clearance and set to be approximately 100 to 200 mm), inthe case where a space between the metal pieces which is firstly formedwhen joining the metal pieces is represented as g.

Further, when heating the metal pieces 1 and 2, in order to prevent thetemperature rise of the member 24 consisting of magnetic substanceitself, the member 24 may be obtained by superimposing a plurality ofinsulated thin plates one above the other. As the member 24, simplesubstance such as iron, nickel, cobalt or others, alloys thereof ornon-crystallized substance can be used in addition to silicon steel.

FIG. 34 shows an example in which the members 24 consisting of magneticsubstance are adapted in the notches u in the joining apparatus providedwith the dislocation preventing plates 8 each having the notches u.

In the apparatus having such a structure, the metal pieces are uniformlyheated over the entire region in the width direction. Also, it isextremely advantageous for completely preventing fluctuations in thevertical direction which are likely to be caused during pressing themetal pieces 1 and 2 against each other.

In the case where there is provided such dislocation preventing plates 8as shown in FIG. 34, if there is provided such a structure as shown inFIG. 35 that each member 24 consisting of magnetic substance can betemporarily removed from each notch u and shifted in parallel with thewidth direction of the metal pieces 1 and 2 to be again inserted in eachnotch u at a predetermined position, it is possible to easily cope withjoining of metal pieces having different widths, thereby performing theeffective continuous hot rolling.

The explanation will now be given as to a case where the overlap widthof the preceding and succeeding metal pieces and the magnetic poles ofthe inductor in the longitudinal direction of the metal pieces areadjusted to improve the heating efficiency at the portions to be joined.

As an inductor used for joining the metal pieces, those having thestructure shown in FIGS. 36 through 38 are typical examples. The stateof magnetic flux distribution when the electric power is supplied tothese inductors to generate magnetic fluxes is as shown in FIG. 39 or40.

As apparent from FIGS. 39 and 40, magnetic fluxes generated between themagnetic poles can be roughly classified into three types irrespectiveof difference between the structures of the inductors.

1) Magnetic fluxes running through the metal pieces in the verticaldirection thereof (region (I)).

2) Magnetic fluxes passing a space between the preceding metal piece andthe succeeding metal piece (region (II)).

3) Magnetic fluxes dispersing without running through the metal piecesand the space therebetween (region (III)).

Among these types of magnetic flux, one contributing to heat the metalpieces is the magnetic flux in (I) and also having a vertical component.In this type of heating method (induction heating method), it istherefore important to secure a large number of magnetic fluxes in (I).

The induced current generated by the magnetic flux vertically runningthrough the metal pieces, namely, the magnetic flux running in thethickness direction intensively flows along the ends of the metalpieces, and this is known as the skin effect. The region in which theinduced current flows is generally defined by a distance from the end ofthe metal piece toward the inside of the metal piece (depth), namely, aso-called osmotic depth d₀ (m) and can be represented by the followingexpression.

d ₀={ρ×10⁷/(μ×f)}^(½)/2π

f: frequency of the alternating magnetic field (Hz)

ρ: electric resistivity (Ω*m)

μ: relative magnetic permeability (−)

Here, in order to find the influence of the osmotic depth determined bya frequency of the alternating magnetic field and the overlap width L(smaller one of two dimensions L1 and L2 shown in FIGS. 36 through 38)of the respective metal pieces and the magnetic poles of the inductor inthe longitudinal direction of the metal pieces upon heating, theexperiment was carried out as follows. The inductor shown in FIG. 36 wasused, and the metal pieces of SUS 304 each having a thickness of 30 mmwere opposedly arranged with a space of 5 mm therebetween. Also, gaps Dbetween the metal pieces and the magnetic poles were made uniform, andheating was carried out while varying the overlap width L of themagnetic poles and the metal pieces. Variations in the temperaturerising speed at that time were observed.

The result is as shown in FIG. 41. Values in FIG. 41 were obtained asfollows. A plurality of double bevel sheath thermometers were embeddedin the metal pieces from the front and rear ends thereof in thelongitudinal direction at a pitch of 3 mm, and the temperature risingspeed was obtained when the current was supplied to the coils of theinductor for three seconds at various alternating magnetic fieldfrequencies (100 Hz through 10 kHz). The temperature rising speed ratio(represented as a mean value of results obtained with frequencies of 100Hz, 500 Hz, 1 kHz and 10 kHz) was arranged with L/d₀=4.0 as a reference.Note that the osmotic depth is approximately 49 mm at 100 Hz, 22 mm at500 Hz, 15 mm at 1 kHz and 5 mm at 10 kHz.

As apparent from FIG. 41, the heating efficiency prominently lowers whenthe overlap width L is 2.0 time as long as the osmotic depth d₀ of theinduced current and thereafter becomes shorter.

That is because, if the overlap width L is not more that 2.0 times aslong as the osmotic depth d₀, it can be considered that the direction offlow of the current induced to the metal is reversed with respect tothat of the current flowing at an inner portion and these currentsweaken their flows each other. Further, the fact that the ratio ofmagnetic fluxes (III) which do not relate to heating of the metal piecesbecomes large and the quantity of effective magnetic fluxes (I) isrelatively decreased can be regarded as another reason.

On the other hand, when the overlap width L is not less than 2.0 timesas long as the osmotic depth d₀, the substantially same temperaturerising speed can be obtained in any case. In the present invention,therefore, it is determined to satisfy L≧2.0*d₀, or more preferably,L≧3.0*d₀ in the relationship between the overlap width L and the osmoticdepth d₀.

As described above, in case of heating and joining the metal pieces, ifthe overlap width L (m) of the rear and front ends of the respectivemetal pieces and the magnetic poles of the inductor in the longitudinaldirection of the metal pieces satisfies L≧2.0×d₀ in the relationshipwith the osmotic depth d₀ of the induced current, an effective heatingcan be carried out.

Referring to FIG. 42, the preceding metal piece 1 and the succeedingmetal piece 2 are opposed to each other with a space g therebetween, andtwo inductors 3 each having a pair of magnetic poles verticallysandwiching the metal pieces are provided thereto to generate analternating magnetic field running through the metal pieces in thethickness direction thereof. The induced current then flows at the endsof the respective metal pieces 1 and 2, and the end region of theopposed faces which will be joined is heated to increase the temperaturetherein by heat generated due to flow of the induced current. In thecase where there is, for example, a temperature difference between thepreceding metal piece 1 and the succeeding metal piece 2, however, sincethe both metal pieces are heated to increase their temperatures at asubstantially equal speed, the temperature difference remains. Thepressing operation may be started while the temperature in one of themetal pieces does not reach a target joining temperature range or,conversely, one of the metal pieces may be excessively heated to befused or melted down. Thus, an excellent joining portion may not beobtained.

In the case where the thickness of the preceding metal piece 1 isdifferent from that of the succeeding metal piece 2, the quantity ofmagnetic fluxes running through the metal pieces in the thicknessdirection thereof can be equal in the both metal pieces, but thetemperature rising speed is faster in the metal piece having a largethickness and slow in the metal piece having a small thickness. In thiscase, the metal pieces are also brought to the above-described state,and an excellent joining state cannot be obtained.

Therefore, in this invention, in case of heating the ends of thepreceding metal piece 1 and the succeeding metal piece 2 to increase thetemperature thereof, the temperatures of the respective metal pieces arefirst measured to grasp the temperature difference therebetween.

Then, as shown in FIG. 43, in order that the temperatures may reach thesame temperature range in the same heating time, the positions of themetal pieces 1 and 2 or the position of the inductor 3 are adjusted (theadjustment of the space g or the positional adjustment of the metalpieces in the longitudinal direction) so that the penetration quantityof the alternating magnetic field is controlled. In this state, thealternating magnetic field running through the metal pieces in thethickness direction thereof is generated and the metal pieces areinduction-heated and, at the same time with or after this heating, atleast one of the clamps 9 moves toward the opposed metal piece to pressthe metal pieces 1 and 2 against each other for joining.

In the case where the rear end of the preceding metal piece and thefront end of the succeeding metal piece move toward each other and thealternating magnetic field running through the metal pieces in thethickness direction thereof is generated to perform induction heating,the rate of temperature rise on heating can be individually controlledin each metal piece when the penetration quantity of magnetic flux isadjusted for each metal piece. Thus, even when the preceding metal pieceis different from the succeeding metal piece in temperature, thicknessor melting point, the both metal pieces are heated to increase theirtemperature to a range suitable for joining the metal pieces insubstantially the same time, and the joining having the substantiallysufficient strength is possible, eliminating such a problem that theplates are ruptured during rolling.

In the case where the inductor 3 such as that shown in FIG. 42 is used,since the density of the magnetic flux generated by the inductor issubstantially constant, the penetration quantity of the magnetic fluxesrunning through each metal piece can be obtained form the product of themagnetic flux density and the area in which the core of the inductorlaps over the metal pieces (which can be calculated from the positionsof inductors 3 a and 3 b and the space g).

For example, in FIG. 43, when heating is carried out assuming that: thearea of the core of the inductor is 76 mm×300 mm=0.0228 m²; the magneticflux density when generating the alternating magnetic field by thisinductor is 0.5 T; the distance between the vertical magnetic poles ofthe inductor is 150 mm; the space g between the preceding metal piece 1(extremely-low carbon steel having a thickness of 28 mm and C of 0.004wt %) and a succeeding metal piece 2 (extremely-low carbon steel havinga thickness of 28 mm and C of 0.004 wt %) is 5 mm; the lap area a of themagnetic poles in the preceding metal piece 1 is 0.01065 m²; the laparea b of the magnetic poles in the succeeding metal piece 2 is 0.01065m²; and the power supply frequency of the alternating magnetic field is1000 Hz, the temperature rising speed of the respective metal pieces isequally 70° C./sec.

In the state of FIG. 44 where the succeeding metal piece 2 is moved backduring heating while the preceding metal piece 1 and the inductors 3 arefixed, when the gap g between the preceding metal piece 1 and thesucceeding metal piece 2 is changed in a range of 0 through 30 mm (whenthe lap area of the inductors is changed), the temperature rising speedratio of the metal pieces shows such a relationship as shown in FIG. 45.If such a relationship is previously grasped, an appropriate penetrationquantity of magnetic flux can be determined in accordance with adifference between the temperatures of the metal pieces or types ofsteel. Further, it can be considered that the temperature rising rate ofthe metal piece is inversely proportional to the thickness of the sameand, if the preceding metal piece 1 is different from the succeedingmetal piece 2 in thickness, the heating and temperature rise may becarried out taking such a relationship into account.

In the present invention, although the penetration quantity of magneticflux in each metal piece is adjusted by moving the metal piece or theinductor, inductors may be individually provided for the respectivemetal pieces to adjust the magnetic flux density of the alternatingmagnetic field itself. It is, however, realistic that the penetrationquantity of magnetic flux is adjusted by relatively moving the metalpiece and the inductor while maintaining the space g between the metalpieces to be constant.

FIGS. 46 through 48 show a specific example of an apparatus having amoving mechanism for adjusting the penetration quantity of magnetic fluxby moving the inductor.

In FIGS. 46 to 48, reference number 25 denotes a frame movable along thelongitudinal direction of the metal pieces 1 and 2; 26, a carrier rollersupporting the metal pieces 1 and 2; 27 and 28, subframes movable alongthe width direction of the metal pieces within the frame 25; 29 and 30,clamps which are provided in the subframes 27 and 28 for clamping andsupporting the metal pieces 1 and 2; 31, a rod which suspends andsupports the inductor 5 provided with a C-shaped core so as to becapable of sliding along the axial direction thereof and which can moveon a rail 1 provided along the width direction of the metal piecestogether with the inductor 5; and 32, a moving mechanism for moving theinductor 5 in the longitudinal direction of the metal pieces separatelyfrom the frame 25. As shown in FIG. 47 illustrating the primary part ofthe moving mechanism 23, the moving mechanism 23 is constituted by fixedwedges 32 a and 32 b held by a suspending/supporting portion of theinductor 5, and movable wedges 32 e and 32 f having hydraulic cylinders32 c and 32 d and provided between the fixed wedges 32 a and 32 b andbetween the rails 1 to face different directions.

There is exemplified the joining apparatus having the above-mentionedarrangement in which the clamps 29 and 30 and the inductor 5 can beeasily removed from the carrier line of the metal pieces duringmaintenance or in an emergency. The adequate heating is enabled byappropriately moving the inductor 5 in the longitudinal direction of themetal pieces by the moving mechanism 32 and adjusting the penetrationquantity of magnetic fluxes.

In FIGS. 47 and 48 showing the primary part of the inductor 5, themovable wedges 32 e and 32 f are first opposedly moved by the hydrauliccylinders 32 c and 32 d to shift the inductor 5. At this time, thesuspending/supporting portions slide on the rails 1, whereby theposition of the inductor 5 is adjusted in the longitudinal direction ofthe metal pieces.

In FIGS. 46 and 48, although there is shown the case where the movementis carried out by using the movable wedges 32 e and 32 f of the movingmechanism 32, one of the movable wedges may employ a spring or a balancecylinder having a function similar to that of the spring. On the otherhand, the inductor 5 may be directly moved by using the hydrauliccylinders instead of the wedges, or the rails 1 themselves may be movedin the longitudinal direction of the metal pieces to shift the inductor5.

In this invention, the description has been given as to the case where apair of inductors, each of which has a pair of magnetic poles verticallysandwiching the metal pieces and which can cover the metal pieces overthe full width thereof, are provided to perform heating and increase thetemperature. However, in the case where a target is metal pieces eachhaving a larger width than that of the inductor, two pairs of, i.e.,four inductors may be provided in the plate width direction of eachmetal piece to perform heating and increase the temperature. In such acase, the heating means is not restricted to a specific form. Further,although the clamps are employed as means used when pressing the metalpieces, pinch rolls may also be adopted.

The explanation will now be give as to a case where a plurality ofinductors are used to heat and join the metal pieces.

In the joining operation which is as shown in FIGS. 49(a) and (b) and bywhich the preceding metal piece 1 and the succeeding metal piece 2 areopposedly provided with a space therebetween to perform heating andincrease the temperature, after the rear end of the preceding metalpiece 1 and the front end of the succeeding metal piece 2 are heated bythe current e induced to the metal pieces by the alternating magneticfield of the inductors 3, the metal pieces are joined by adding thepressing force thereto along the longitudinal direction of the metalpieces. However, the area in which the induced current flows isdecreased as the width of the respective metal pieces increases,resulting in the reduction in the temperature rising efficiency at endportions.

As a countermeasure, turning on the electricity for a long time orincrease in the quantity of heat input can be considered. When the timefor turning on the electricity is prolonged, however, the portiondirectly below the inductor is heated for a long time, whereby aburn-through is caused due to an excessive heating. On the other hand,when the quantity of heat input is tried to be increased, a maximumvalue of the input power for one inductor is 2000 through 3000 kW whichis the limit at a stage of manufacturing the inverter, and there ishence a limitation.

Accordingly, in order to solve the above-mentioned problems, as shown inFIG. 50, it is required to provide a plurality of inductors along theportions which will be joined in the metal pieces and, for example, twoinductors must be provided on a working side (WS) and a driving side(DS).

As a power supply of the inductor, since an inverter whose oscillationfrequency depends on a load impedance as viewed from the inverter side,namely, a self-controlled inverter is frequently adopted, when the loadimpedance on the WS is different from the load impedance on the DS asviewed from the inverter, the oscillation frequency varies.

In the actual joining operation, the joining state on the WS is oftendifferent from that on the DS. In the case where the load impedance ischanged depending on the joining state (the frequency actually varies atthe time of occurrence of an arc), the oscillation frequency of theinverter on the WS is thus different from that on the DS.

If the oscillation frequency of the inverter on the WS is different fromthat on the DS, phases of the WS and DS are inverted (for example, incase of 500 Hz and 510 Hz, the phases are inverted after approximately0.98 second), and the induced currents induced to the metal piecescancel out. As a result, the temperature rising efficiency is decreased,and a desired heating performance cannot be obtained.

In the invention, as shown in FIGS. 51(a) and (b), inverters areexclusively connected to inductors L10 and L20 for use, respectively,and phases are synchronously controlled so that the currents having thesame phase flow in the inductors L10 and L20. Eddy currents L11, L21,L12 and L22 induced to the metal pieces have the same phase to realizethe effective heating by the induced currents.

FIG. 52 is a block diagram showing a control system in the case where apair of inductors 3 for generating the magnetic field are provided inthe width direction of the metal pieces.

In the drawing, reference numeral 33 denotes an electric power commandapparatus; 34 and 35, inverters; and 36, a phase control circuit.

As shown in FIG. 52, the inverter 34 is of a self-controlled type, andits oscillation frequency is determined by a circuit constant (thisfrequency varies depending of the joining state such as a joining lengthof the metal pieces). In addition, the phase control circuit 36 has afunction for detecting an oscillation frequency and a phase of theinverter 34 and generating ignition pulses which are given to theinverter 35. Note that the inverter 35 is of an externally-controlledtype and its oscillation frequency is determined by the ignition pulsessupplied from the phase control circuit 36.

In FIG. 51(b), assuming that the inductance is Li when the loads areviewed from capacitors C, each inverter oscillates by a resonancefrequency fi represented by the succeeding expression:

fi=1/{2π(LiC)^(½)}

Li is determined by the shape of the metal pieces, the distance betweenthe inductors and the metal pieces and the space between the metalpieces, and thus determined by Li=Li0+(Mi1²/Li1)+(Mi2²/Li2) from Li0 ofthe inductors, Li1 of the preceding metal piece, Li2 of the succeedingmetal piece, and inductances Mi1 and Mi2 between the inductors and thepreceding and succeeding metal pieces.

In this case, when the electricity is turned on for five seconds, adifference between the oscillation frequencies such that there may be noproblem is 0.1 Hz.

In order to synchronize the oscillation frequencies and the phases ofthe inverters on the WS and DS, such a structure as that shown in FIG.53 may be adopted. That is, the self-controlled inverters are used aspower supplies on both the WS and the DS, and the inverters areconnected on the secondary sides. The impedances viewed from theinverters thus become common, thereby synchronizing the oscillationfrequencies and the phases of the inverters on the WS and DS.

In connection with the above-described case where a plurality inductorsare used, a point in arranging each inductor will now be mentioned.

In order to simply and rapidly heat and join the preceding metal pieceand the succeeding metal piece without consuming unnecessary energy, itis particularly preferable to adopt a so-called transverse inductionheating method by which the preceding and succeeding metal pieces areopposed with a gap of a few or tens mm at their ends (the front end andthe rear end), and the inductor 3 having such a structure as that shownin FIG. 54 is disposed herein to perform heating. According to thismethod, as shown in FIG. 55 which is a top plan view of a joining regionof the metal pieces, the induced current e flows at the end of eachmetal piece, and the temperature in the portions which will be joined inthe metal pieces is precedently increased by heating due to this currente, thereby enabling easy joining of the metal pieces by the pressingoperation which is subsequently performed. In such a joining method,however, there is no problem in particular when the width of themagnetic poles of the inductor is larger enough than that of the metalpieces, though in the case where the metal pieces having a width largerthan that of the magnetic poles of the inductor are joined, thepenetration quantity of magnetic fluxes are decreased at the width endsof the metal pieces, and hence the induced current cannot flow in theentire end region of the opposed face of the metal pieces. As a result,the temperature distribution in the width direction becomes uneven,thereby making it difficult to realize the secure joining. On the otherhand, in the case where the width of the magnetic poles is made large incorrespondence with the width of the metal pieces, it is necessary toincrease the current supplied to the inductors in order to obtain asatisfactory quantity of magnetic fluxes running through the metalpieces per unit area. Further, the current supplied to the inductorsmust be restricted to be within such a range that coils or coresconstituting the inductors or the metal pieces and in particular thecentral region thereof cannot be melted down by Joule heat, and theelectric power which can be input to the inductors therefore has alimitation. After all, even though the width of the magnetic poles isenlarged, the quantity of magnetic fluxes per unit area becomes uneven,and the entire area at the ends of the metal pieces cannot be heated atthe same speed.

In order to eliminate such a problem, it is extremely effective to use aplurality of inductors as shown in FIG. 50.

However, if a plurality of inductors are only provided along the widthdirection of the metal pieces at their portions which will be joined,when the space provided between the magnetic poles of the inductorsadjacent to each other is large, the temperature rising speed in themetal piece at its portion corresponding to this space is slow ascompared with that in any other region. Therefore, there is adisadvantage that even when the heating temperature reaches a targetvalue, the temperature at this portion is lower than this value.

When heating is continued until the temperature in the region whichcorresponds to the space between the magnetic poles and where thetemperature rising speed is lowered reaches a temperature at whichjoining is possible, any other potion is melted down, disadvantageouslyaffecting on the joining operation for the metal pieces.

In order to eliminate the above-mentioned problems when heating themetal pieces by using a plurality of inductors, the space is preferablyset to be not more than 5 times as long as the osmotic depth d₀ (m) ofthe induced current which can be represented by d₀={ρ×10⁷/(μ×f)}^(½)/2π(it is preferable to provide no space between the magnetic poles of theinductors, namely, the space should be 0, but a protective cover andothers are actually disposed to each inductor and the space cannot beset to 0). Here, f represents the frequency of the alternating magneticfield (Hz); ρ, the electric resistivity of the metal pieces (Ω*m), andμ, the relative magnetic permeability.

FIG. 56 shows an example in which a pair of inductors 3 a and 3 b arearranged at the ends of the metal pieces 1 and 2 along the widthdirection thereof. When the inductors 3 a and 3 b are provided as shownin the drawing and a space w₁ between the adjacent magnetic poles is setwithin the above-described range, the induced currents generated by therespective inductors 3 a and 3 b are united to have a large value. Theuniform heating is consequently realized in the full width direction.

A pair of inductors are provided in the width direction of the metalpieces and only the space between the magnetic poles of the inductorsadjacent to each other is varied, and FIG. 57 shows the result ofresearch of influences by a ratio of the space to the osmotic depth d₀,the research having been carried out in accordance with the succeedingpoints:

1) Assuming that the portions which will be joined and correspond withthe magnetic poles are 100%, a ratio of the temperature rising speed inthe portion corresponding to the space between the magnetic poles; and

2) In the width direction of the portions to be joined, a length of aregion where the temperature rising speed is not more than 90% withrespect to that in the portion corresponding with the magnetic poles.

In this case, the joining conditions are such that: the width of themetal pieces is 1500 mm; the width of the magnetic poles, 1000 mm×2; theapplied power, 1000 KW; and the alternating magnetic field frequency, 1KHz.

As apparent from the drawing, if the space between the magnetic poles isnot less than five times as long as the osmotic depth d₀, thetemperature rising speed in a portion corresponding with the spacelowers below 90% of the temperature rising speed of the portioncorresponding with the magnetic poles, and a problem may be caused inthe actual operation. On the other hand, the region in which thetemperature rising speed is not more than 90% gradually increases from0. Therefore, in the case where a plurality of inductors are used toheat the metal pieces and increase the temperature for joining, it ispreferable to set the space between the magnetic poles adjacent to eachother to be not more than five times as large as the osmotic depth d₀ inconnection with the frequency of the alternating magnetic field.

FIG. 58 shows a structure of an apparatus for carrying out such aheating operation.

FIG. 58 shows an example in which two inductors are arranged along thewidth direction of the metal pieces 1 and 2, and projections 37 areprovided to the respective inductors 3 a and 3 b on the adjacent facesof the magnetic poles adjacent to each other so that the respectiveinductors 3 a and 3 b can be brought into contact with each other or aspace provided therebetween can be narrowed.

The description has been given as to how to uniformly heat the entirearea of the metal pieces in the width direction thereof. As to theabove-mentioned various conditions, even if they are satisfied, when themetal pieces joined at a heating temperature of approximately 1350through 1400° C. are rolled by a finishing rolling mill composed of aplurality of stands with a draft percentage increased by ten times ormore, it may not be said that rolling with respect to all kinds of steelcan be performed without causing a rupture at the joined portion untilthe completion of rolling.

For example, in regard of extremely-low carbon steel of SS400 or thathaving a carbon content of not more than 100 ppm, there sometimes occursa problem that the plate is ruptured during rolling.

Irrespective of types of steel, it is necessary to provide a heatingcondition that such a problem as rupture of plate during finishingrolling which is thereafter performed is not caused, and such a heatingcondition will be described hereinbelow.

In order to realize an excellent joining of the metal pieces, whencarrying out joining, the temperature of the portions to be joined mustbe increased to a value such that the oxidized scale on the surface canbe melted and removed, or the base metal can be melted at least on theopposed end faces of the portions which will be joined. Either of themmust be satisfied.

The present inventors, therefore, researched about the joiningconditions that the metal pieces can withstand rolling with the draftpercentage increased by not less than five times after joining, andparticularly about a preferable range of heating temperature, forvarious kinds of carbon steel having a carbon content of 1.3 wt % or 5ppm.

The quality of joining was judged in accordance with existence/absenceof rupture in the plate during finishing rolling after joining and thejoined state obtained after rolling. In the judgment of quality, therewas actually no problem when finishing rolling was carried out withoutany problem or even when a crack at a joined portion was partiallygenerated after rolling, and the joining can be hence said to beexcellent.

The obtained result is illustrated in FIG. 59.

As shown in the drawing, it was found that the preferable range ofheating temperature largely varied in accordance with the carbon contentand was extending with values of temperature higher than 1350 through1400° C., values of which were conventionally good, when the carboncontent was small and, on the other hand, the preferable range ofheating temperature was extending with lower values when the carboncontent was large.

In this case, it was discovered that the preferable range of heatingtemperature would be excellently appeared when a melting temperature ofthe scale of iron oxide, a solidus line temperature of the metal piecesand a liquidus line temperature of the metal pieces were used asparameters.

FIG. 60 shows the relationship between the carbon content of the metalpieces and the liquidus and solidus line temperatures of the metalpieces (this figure is drawn by using computational expressionsdescribed in “Handbook of Iron and Steel”, 3rd edition, Basic I, P. 205(Maruzen Co., Ltd.)). Further, the drawing also shows the meltingtemperature of the oxidized scale and the rolling state obtained in FIG.59.

As apparent from a comparison between FIGS. 59 and 60, an optimum rangeof heating temperature was able to be excellently appeared by using thesolidus line temperature of the metal pieces (T_(S)) and the liquidusline temperature of the metal pieces (T_(L)), and finishing rolling wascarried out without any problem if the temperature (T) at the portionswhich will be joined satisfied a range represented by the succeedingexpression:

T _(S) ≦T≦(T _(S) +T _(L))/2

where T_(S) indicates a solidus line temperature of the metal pieces (°C.), and

T_(L) represents a liquidus line temperature of the metal pieces (° C.).

Although the above range of temperature is the optimum range of heatingtemperature having no problem, the preferable range of heatingtemperature actually having no problem can be represented as follows.

That is, the range can be precisely expressed by using the solidus linetemperature of the metal pieces (T_(S)), the liquidus line temperatureof the metal pieces (T_(L)) and the melting temperature of the scale ofiron oxide (T_(c)) as parameters, and the following preferable ranges ofheating temperature were found out. Namely, if the solidus linetemperature of the metal pieces (T_(S)) is equal to or above the meltingtemperature of the scale of iron oxide (T_(c)), the temperature in theportions to be joined (T) is in such a range that is higher than anintermediate temperature of the melting temperature of the scale of ironoxide (T_(c)) and the solidus line temperature of the metal pieces(T_(S)) and lower than an intermediate temperature of the solidus linetemperature (T_(S)) and the liquidus line temperature (T_(L)) of themetal pieces, namely, a range represented by the succeeding expression:

(T _(c) +T _(S))/2≦T≦(T _(S) +T _(L))/2

On the other hand, if the solidus line temperature of the metal pieces(T_(S)) is lower than the melting temperature of the scale of iron oxide(T_(c)), the temperature in the portions to be joined (T) is in such arange that is higher than the solidus line temperature of the metalpieces (T_(S)) and lower than an intermediate temperature of the solidusline temperature (T_(S)) and the liquidus line temperature (T_(L)) ofthe metal pieces, namely, a range represented by the succeedingexpression:

T _(S) ≦T≦(T _(S) +T _(L))/2

It was confirmed that T_(S) and T_(L) were changed to some extentdepending on a main component, but the excellent joining was realizedirrespective of types of steel if the above temperature conditions weresatisfied.

Accordingly, in the present invention, in the case where the both metalpieces are joined by heating and pressing the rear end of the precedingmetal piece and the front end of the succeeding metal piece in a hotrolling line, the pressing operation is effected under such atemperature condition that at least the temperature T (° C.) at theportions to be joined satisfies the succeeding expression:

T _(S) ≦T≦(T _(S) +T _(L))/2

or under such a temperature condition that the same satisfies thesucceeding expressions:

(1) ifT_(c) ≦T _(S),

(T _(c) +T _(S))/2≦T≦(T _(S) +T _(L))/2, and

(2) if T _(c>T) _(S)

T _(S) ≦T≦(T _(S) +T _(L))/2

Pressing operation for joining the preceding metal piece and thesucceeding metal piece needs to be performed to such an extent that thesufficient strength can be obtained. In the case where cutting iscarried out using a crop shear, the vertical cross section of the metalpieces in the longitudinal direction thereof is as shown in FIG. 61, andit is preferable to secure a pressing quantity of approximately 8through 10 mm in order that the preceding metal pieces and thesucceeding metal pieces having such a cross section may beinduction-heated to be firm.

BEST MODE FOR CARRYING OUT THE INVENTION EMBODIMENT 1

Embodiment (1)

Sheet bars (low carbon steel) each having a width of 1000 mm, athickness of 30 mm and a temperature of 1000° C. were joined by using anequipment (provided with dislocation preventing plates of SUS304 eachhaving a thickness of 40 mm and 20 notches each having a width of 30 mmand a length of 900 mm (a width at a portion for suppressing deformationof the plate is 20 mm), and a finishing roller mill group having sevenstands) such as shown in FIG. 20 under the succeeding conditions, andhot finishing rolling for obtaining a finished plate width of 3 mm wascarried out. Further, the temperature distribution of the sheet bars inthe width direction thereof at the time of completion of joining and thestate of rupture of the plates during rolling were examined.

Conditions:

a. The distance between a preceding sheet bar and a succeeding sheetbar: 5 mm.

b. The size of a core of an inductor: width=1000 mm, dimension along thelongitudinal direction of the sheet bars=240 mm.

c. The power supplied to the inductor: 1000 kw, and a frequency: 650 Hz.

d. Ten constitutive members having a size of a=200 mm, b=20 mm and c=20mm (see FIG. 62) are disposed to a space between the inductor and thesheet bars at an interval of 50 mm along the plate width direction, andeach reverse magnetic field generation circuit positioned at a centralportion in the plate width direction (the circuit inwardly positionedfrom the width end by 250 mm) is closed to perform heating for 12seconds and increase the temperature.

e. The pressing force: 2 kg/mm² (pressed after heating and increasingthe temperature).

As a result, it was confirmed that the temperature distribution in theplate width direction was improved as shown in FIG. 63 and the stablehot rolling was enabled without causing rupture of the plates duringrolling.

Embodiment (2)

Sheet bars (low carbon steel) each having a width of 1000 mm, athickness of 30 mm and a temperature of 1000° C. were joined by using anequipment (provided with dislocation preventing plates of SUS304 eachhaving a thickness of 40 mm and 20 notches each having a width of 30 mmand a length of 900 mm (a width at a portion for suppressing deformationof the plate is 20 mm), and a finishing roller mill group having sevenstands) such as shown in FIG. 20 under the succeeding conditions, andhot finishing rolling for obtaining a finished plate width of 1.2 mm wascarried out. Further, the temperature up speed ratio in the widthdirection of the sheet bars at the time of heating and the state ofrupture of the plates during rolling were examined.

Conditions:

a. The distance between a preceding sheet bar and a succeeding sheetbar: 5 mm.

b. The size of a core of an inductor: width=1000 mm, dimension along thelongitudinal direction of the sheet bars=240 mm.

c. The power supplied to the inductor: 1000 kw, a current: 6120 A, afrequency: 650 Hz, and a magnetic flux density; 0.21 T.

d. Constitutive members each including magnetic substance (formed bysuperimposing 70 thin plates of silicon steel having an insulating filmone on another) and having a size of a=200 mm, b=1 mm and c=20 mm (seeFIG. 62) are provided to notches of the dislocation preventing plate(see FIG. 11) at a space between the inductor and the sheet bars, andall the reverse magnetic field generation circuits are opened for eightseconds from the start of heating, and a circuit positioned at a centralportion in the plate width direction (the circuit inwardly placed at aposition distanced from the width end by 250 mm) is closed to performheating for 2 seconds.

e. The pressing force: 50 tons (pressed after heating and increasing thetemperature).

FIG. 64 is a graph in which the temperature rising speed ratios arecompared, and it was confirmed that the effective heating was enabledand no rupture of the plates occurs during rolling in the case where themembers consisting of magnetic substance were provided as theconstitutive members.

EMBODIMENT 2

Embodiment (1)

An apparatus such as shown in FIG. 21 was used on the entry side of therolling equipment for hot rolling, and joining of metal pieces havingthe same width dimension was tried. Both the preceding metal piece andthe succeeding metal piece are extremely-low carbon steel and have athickness of 30 mm, a width of 800 mm and a length of 6000 mm.

These preceding and succeeding metal pieces were opposed with a space of5 mm being formed therebetween, and copper plates as conductive memberswere provided at the both width ends of the metal pieces in closeproximity with each minute gap of 4 mm therebetween. Further, thealternating current was supplied to such an inductor as shown in thedrawing, and the alternating magnetic field running through the magneticpieces in the width direction thereof was generated to perform heating.

A temperature of the metal pieces before heated is 1000° C.; athickness, a width and a length of each copper plate extending over thepreceding metal piece and the succeeding metal piece are 30 mm, 200 mmand 600 mm, respectively; and a length and a width of each core of theinductor in cross section parallel with the metal pieces are 240 mm and1000 mm, respectively.

A distance between each magnetic pole and the metal pieces is 90 mm onthe upper side and 90 mm on the lower side; a frequency of thealternating magnetic field is 1000 Hz; and an input power is 980 kW.

After the induction heating was carried out for ten seconds under theseconditions, the preceding and succeeding metal pieces were opposed toand pressed against each other using the clamps with the pressing forceof 40 t applied to complete the joining. As a comparative example, themetal pieces were joined under the same conditions with those of theembodiment except that the copper plates were not provided to the sidesof the both metal pieces.

FIG. 65 shows a result of measuring the temperature rising speeds at theportions to be joined in the embodiment and the comparative example (thetemperature was measured by embedding double bevel thermometer at aposition inwardly distanced from the end of each metal piece by 1.5 mmin the longitudinal direction thereof).

FIG. 65 illustrates the temperature rising speed ratio in the vicinityof the end of each metal piece in the width direction thereof assumingthat a central portion of the metal piece in the width direction is 1.

As obvious from the drawing, when the conductive member was provided tothe side of each metal piece in close proximity according to the presentinvention, the temperature up speed at the end in the width directionapproximated the temperature rising speed at the central portion. Thatis because the induced current generated directly below a core of theinductor flows to the end of the metal pieces in the width direction.Thus, the end of each metal piece in the width direction was heated andsoftened at a temperature such that the joined portion having asufficient strength was able to be obtained without causing the meltdown in the central region in the width direction. Thereafter, when therolling was carried out, it was confirmed that no cracks were developedat the joined portion to bring about the rupture and the excellentrolling was enabled.

Embodiment (2)

As shown in FIG. 23, individual copper plates were provided to the bothwidth ends of the preceding metal piece and the both width ends of thesucceeding metal piece in close proximity. A thickness, a width and alength of each copper plate were 30 mm, 200 mm and 300 mm, respectively,and a gap between the respective metal pieces and the respective copperplates was all 4 mm. In regard of other conditions, the type and size ofsteel used as the preceding and succeeding metal pieces, the size of aninductor, an input power, a frequency and other were all the same withthose in Embodiment (1).

When the temperature rising speed in the vicinity of the portions of themetal pieces which will be joined was measured during heating and thetemperature rising speed ratio of the both width ends with respect tothat of the central portion in the width direction was calculated, theresult equal to that of Embodiment (1) was obtained. No cracks wasgenerated and developed by the rolling thereafter performed, and theplates were excellently joined.

Embodiment (3)

In this embodiment, an examination was carried out about a case wherethe current whose phase is the same with that of the induced current wassupplied to the conductive members.

By using such an inductor as shown in FIG. 25, the preceding andsucceeding metal pieces both having a width of 800 mm were joined. Atthe time of heating, the inductor having a substantially-C-shaped core(a sectional dimension parallel with the metal pieces: a length=1000 mm,a width=240 mm) was used so that the copper plates which are theconductive members overlap on the magnetic poles of the inductor.

The preceding metal piece and the succeeding metal piece were of SUS 304steel type and had a temperature of 900° C. before heating, and thecopper plates were provided to the both width ends of the metal piecesin close proximity so as to connect the rear end of the preceding metalpiece to the front end of the succeeding metal piece.

Each of the copper plates had a thickness of 30 mm, a width of 200 mmand a length of 600 mm. Further, a frequency of the alternating magneticfield in a range of 500 Hz to 10 kHz was set to 500 Hz, 1 kHz and 10kHz, and an input power was 780 kW. Under these conditions, theinduction heating was performed for ten seconds and the preceding metalpiece and the succeeding metal piece were then pressed against eachother with a pressing force of 40 t by using clamps to complete thejoining.

The temperature rising speed in the vicinity of the joining portion ofthe succeeding metal piece was measured during heating in eachfrequency, and the temperature rising speed ratio of the end portion inthe width direction with respect to that of the central portion in thewidth direction was obtained as a mean value.

As a result, the temperature rising speed at the end portion in thewidth direction further approximated to the temperature rising speed inthe central portion as compared to Embodiment (1).

Thus, the preceding metal piece and the succeeding metal piece was ableto be joined in the entire region in the width direction thereof, and nocracks were developed from the joined portion to lead to rupture in therolling thereafter performed.

EMBODIMENT 3

Embodiment (1)

In order to join the preceding metal piece and the succeeding metalpiece both having a plate thickness of 30 mm and a plate width of 800 mmand being of low carbon steel type, heating (conditions for heating andincreasing the temperature: a longitudinal dimension of a core=240 mm, awidth dimension of the core=1000 mm, a gap between upper and lowermagnetic poles=210 mm, an input power=200 Kw and alternating magneticfield frequency=2000 Hz, a conductive member: material=graphite, athickness=30 mm, a width=200 mm and a length=250 mm, and four conductivemembers were pressed against the preceding and succeeding metal piecesat four width end portions) and pressing (pressing condition: pressingforce of 60 ton) were carried out in the state shown in FIG. 28 to jointhe both metal pieces. The obtained metal piece was supplied to a hotrolling equipment to be subjected to finishing rolling, and rupture ofthe plate which might be caused by rolling was checked.

As a result, it was confirmed that there was no rupture of the platecaused due to cracks at a joined portion.

EMBODIMENT 4

Embodiment (1)

After the sheet bars (low carbon steel) each having a plate width of1000 mm and a thickness of 30 mm were joined by using an equipmentprovided with an apparatus having such a structure as shown in FIG. 34under the succeeding conditions, hot finishing rolling for obtaining aplate having a finished plate thickness of 3 mm was carried out, andexaminations were made into an existence/absence of rupture of the plateduring rolling and a temperature distribution in the plate widthdirection immediately after joining of the sheet bars.

Conditions for joining the sheet bars:

a. A temperature of the sheet bars before finishing rolling isapproximately 900 through 1000° C., and an electric resistivity in thistemperature range is approximately 120×10⁻⁸ Ωm. Therefore, when thealternating magnetic field was applied with a frequency of 500 Hz, theosmotic depth d₀ is approximately 25 mm. Each member consisting ofmagnetic substance (dimension: a width of 100 mm×a length of 150 mm×aheight of 30 mm) was thus disposed at a position distanced from thewidth end of the plate by 10 to 110 mm.

b. A space between a preceding sheet bar and a succeeding sheet bar: 10mm.

c. Size of an inductor (core): an inductor having a dimension along thewidth direction of the sheet bars of 1000 mm and a dimension along thelongitudinal direction of the same of 240 mm was used.

d. A power input to the inductor: 1500 kW, a frequency: 500 Hz.

e. A heating time: 10 seconds.

f. A pressing force: 3 kg/mm².

FIG. 66 shows the result of comparison between the temperaturedistributions in the width direction of the sheet bar before and afterheating when the both sheet bars were joined in accordance with thepresent invention, and FIG. 67 shows the temperature distributions whenthe usual heating was performed (a comparative example: when membersconsisting of magnetic substance were not disposed).

As apparent from FIGS. 66 and 67, it was confirmed that the applicationof heat to the metal pieces extremely reduced portions in which theapplication of heat was insufficient which would lead to an existence ofnon-joined portions.

FIG. 68 shows a comparison of calorific power ratios on the joined faceof the sheet bar.

EMBODIMENT 5

Embodiment (1)

Hot-roughed sheet bars (900° C.) of extremely-low carbon (C=20 ppm)steel each having a width of 1000 mm and a thickness of 40 mm wereopposed to each other with a space of 5 mm therebetween; inductors (awidth and a length of each magnetic pole opposed to the sheet bars was240 mm and 1000 mm, respectively) shown in FIG. 37 were arranged at therear and front ends of the opposed sheet bars with a distance D to thesheet bars being 120 mm and an overlap width L of the sheet bars andeach magnetic pole being 70 mm; and the alternating magnetic field wasgenerated by the inductors with an input power of 1350 kW and afrequency of 650 Hz to perform heating. In this case, the osmotic depthwas 22 mm; the temperature rising speed was 70° C./s; and a timesrequired to reach a target heating temperature was approximately 10seconds.

Embodiment (2)

Hot-roughed sheet bars (950° C.) of high carbon (C=0.75%) steel eachhaving a width of 1000 mm and a thickness of 30 mm were opposed to eachother with a space of 10 mm therebetween; inductors (a width and alength of each magnetic pole opposed to the sheet bars was 100 mm and1200 mm, respectively) shown in FIG. 38 were arranged at the rear andfront ends of the opposed sheet bars with a distance D to the sheet barsbeing 60 mm and an overlap width L of the sheet bars and a coil being 45mm; and the alternating magnetic field was generated by the inductorswith an input power of 1000 kW and a frequency of 650 Hz to performheating. In this case, the osmotic depth was 22 mm; the temperaturerising speed was 70° C./s; and a times required to reach a targetheating temperature was approximately 7.5 seconds.

EMBODIMENT 6

Embodiment (1)

In order to join the preceding metal piece having a width of 600 mm, athickness of 28 mm and a melting point of 1532° C. (a type of steel:extremely-low carbon steel having a carbon content of 0.002 wt %) andthe succeeding metal piece having a width of 600 mm, a thickness of 28mm and a melting point of 1485° C. (a type of steel: carbon steel havinga carbon content of 0.7 wt %) to each other, after heating andincreasing the temperature under such conditions that: an area A of acore of an inductor was 76 mm×300 mm=0.0228 m²; a magnetic flux densityof the alternating magnetic field was 0.5 T; a distance between upperand lower magnetic poles of the inductor was 150 mm; a temperature ofthe preceding metal piece before starting heating was 1000° C.; atemperature of the succeeding metal piece before starting heating was1000° C.; a space g between the preceding and succeeding metal pieceswas 5 mm; a lap area a of each magnetic pole in the preceding metalpiece was 0.01050 m²; a lap area b of each magnetic pole in thesucceeding metal piece was 0.01080 m²; a frequency of the alternatingmagnetic field was 1000 Hz; two inductors were prepared and disposed atthe both width ends of the metal pieces; a heating time was 6.5 seconds;and a pressing force was 2 kgf/mm² (bearing), the both metal pieces werepressed against and joined to each other, and an examination was madeinto a tensile strength at the joined portion after completely cooled.

In regard of heating of the metal pieces under the above-mentionedconditions, it was confirmed that the temperatures of the precedingmetal piece and the succeeding metal piece after heating reached 1475°C. and 1430° C. which were lower than the respective melting points by55° C., respectively. Further, as to the tensile strength of the joinedportion, there was obtained an excellent result, i.e., 28 kgf/mm²corresponding to approximately 90% of a tensile strength (31 kgf/mm²) ofthe preceding metal piece.

EMBODIMENT 7

Embodiment (1)

An apparatus shown in FIG. 51 in which two inductors (the size of a coreof each inductor: a length of 240 mm×a width of 1000 mm) for generatingthe magnetic field were provided in the width direction of the metalpieces was used; the electric power of 1000 kW was input to eachinductor while synchronously controlling phases; and metal pieces (900°C.) of regular steel each having a width of 1000 mm and a thickness of30 mm were heated and joined to each other.

As a result, in the conventional joining, since the induction currentsinduced to the metal pieces canceled out depending on the widthdimension of the metal pieces, the temperature rising efficiency waslowered, whereby a desired heating efficiency might not be obtained. Inthe case where two inductors were synchronously operated in accordancewith the invention, however, it was confirmed that even when the widthof the metal pieces was large, the temperature rising efficiency was notdecreased, thereby enabling the stable joining.

EMBODIMENT 8

Embodiment (1)

In the hot rolling line shown in FIG. 20, after the sheet bars (lowcarbon steel) each having a width of 1800 mm and a thickness of 30 mmwere opposed to each other as the preceding metal piece and thesucceeding metal piece with a space of 10 mm therebetween, heating wascarried out for 10 seconds and the temperature was increased to 1500° C.by a joining apparatus having inductors whose structure is as shown inFIG. 58 (a width of each magnetic pole was 800 mm; a length of eachmagnetic pole was 250 mm; and a distance between adjacent magnetic poleswas 75 mm) under such conditions that an input power was 1500 kW and afrequency was 500 Hz, and the metal pieces were pressed against eachother by a force of 3 kgf/mm², thereby completing the joining of theboth metal pieces. The obtained metal piece was then rolled by a hotfinishing mill having seven stands until the plate thickness of 3 mm wasrealized. At this time, an existence/absence of rupture in the joinedregion was checked, but the plate had no rupture and the excellentcontinuous rolling was hence performed.

On the other hand, in the case where an apparatus such as shown in FIG.70 in which two inductions each having a C-shaped core are arranged inthe width direction of the metal pieces was used, the succeeding factswere found out.

That is, since the temperature of the metal pieces is usually in a rangeof 900 through 1000° C. during joining and the electric resistivity ρ ofthe metal pieces is approximately 120×10⁻⁸ Ω*m in this range oftemperature irrespective of types of steel, when the induction heatingwas effected with a frequency of the alternating magnetic field of 500Hz, the calculated osmotic depth d₀ becomes approximately 25 mm. Inheating using the apparatus shown in FIG. 70, a limit of a space betweenthe magnetic poles is 150 mm (d₀×6) from a viewpoint of an insulatinglimit of two inductors adjacent to each other and, as shown in FIG. 71illustrating the heating quantity distribution in the width direction ofthe metal piece, the heating quantity at a space between the magneticpoles did not exceed 90% of the that in the joining region correspondingwith the magnetic poles, resulting in the insufficient joining at thisregion.

FIG. 72 shows a distribution of heating quantity in the case where anapparatus having inductors shown in FIG. 58 was used, it is obvious thatthe a rate of decrease in the heating quantity at a portion between theinductors is extremely low as compared to data shown in FIG. 70.

Although this embodiment has been described as a case where a space (adimension between two projections) between magnetic poles adjacent toeach other is 75 mm (usually, a value of 150 mm is a lower limit eventhough the space is narrowed as possible), any apparatus, for example,one shown in FIG. 73 in which the space is 0 mm can be applied if only adesired heating capacity can be secured.

EMBODIMENT 9

Embodiment (1)

A joining apparatus utilizing heating by the induced current wasprovided between a delivery side of a roughing mill and an entry side ofa finishing mill in a hot rolling line; the rear end of the precedingmetal piece and the front end of the succeeding metal piece were cut toobtain desired end shapes by a crop shear at an earlier stage ofoperation in the joining apparatus; heating was carried out at varioustemperatures by the induced current; the metal pieces were pressedagainst and joined to each other; and the thus-obtained metal piece wasthen supplied to a finishing mill.

The temperature rising speed was previously set to be 100° C./s, and thetemperature of each roughed sheet bar immediately before heating wasalso adjusted in accordance with a heating furnace extract temperatureand a roughing speed to be 1000° C.±20° C.

In regard of a steel type, carbon steel having a carbon content of 20ppm or 1.3 wt % was used.

Rolling conditions were such that: a sheet bar width after roughing was700 to 1900 mm; a sheet bar thickness was 25 to 50 mm, and a steel platethickness on a delivery side of a finishing mill on a seventh stage was0.8 to 3.5 mm.

The inductor for generating the alternating magnetic field was disposedin such a manner that a pair of magnetic poles would vertically sandwichthe front and rear ends of the metal pieces and the magnetic field couldact upon the entire region of the portions to be joined. The electricpower was supplied from the same alternating power supply to thevertically extending inductor, and a maximum input power capacity was3000 kW.

In this case, the heating processing was carried out under such acondition that a solidus line temperature (T_(S)) and a liquidus linetemperature (T_(L)) were obtained from components of the metal piecesand an ultimate temperature of heating T satisfiedT_(S)≦T≦(T_(S)+T_(L))/2. Note that a melting temperature T_(c) of thescale in the embodiment was 1350° C.

The obtained result is shown in Table 1.

TABLE 1 Content Plate Plate Solidus Result of C width thickness ratio TT_(S) T_(L) T_(S) + Draft of No. (wt %) (mm) (mm) (%) (° C.) (° C.) (°C.) T_(L)/2 (%) rolling 1 1.3 700 30 94 1310 1300 1460 1380 15.0 good 20.8 1000 30 43 1430 1375 1472 1423 10.0 good 3 0.2 1600 30 85 1470 14601525 1492 15.0 good 4 0.05 700 30 solid 1515 1510 1532 1521 10.0 good 50.05 1500 25 100  1510 1510 1532 1521 31.3 good 6 0.05 1500 40 77 15151510 1532 1521 15.0 good 7 0.002 1900 25 87 1522 1520 1535 1527 31.3good 8 0.002 700 50 53 1527 1520 1535 1527 20.0 good

As apparent from the table, when the heating/joining processing waseffected under the conditions satisfying an optimum heating temperaturerange, good finishing rolling was able to be performed in any case.

Embodiment (2)

A type of steel and rolling conditions are the same with those inEmbodiment (1).

Heating was carried out under such a condition that a meltingtemperature of scales of iron oxide (T_(c)), a solidus line temperature(T_(S)) and a liquidus line temperature (T_(L)) were obtained fromcomponents of the metal pieces and an ultimate temperature of heating Tsatisfied (T_(c)+T_(S))/2≦T≦(T_(S)+T_(L))/2. However, in case ofT_(c)>T_(S), the condition was such that T_(S)≦T≦(T_(S)+T_(L))/2 wassatisfied.

The obtained result is shown in Table 2.

TABLE 2 Content Plate Plate of C width thickness T T_(C) T_(S) T_(L)Draft Result of No. (wt %) (mm) (mm) (° C.) (° C.) (° C.) (° C.) (%)rolling  9 1.3 700 30 1375 1365 1300 1460 10.0 good 10 0.7 900 30 13721365 1377 1485 20.0 partially incom- plete joining after rolling 11 0.21300 30 1470 1360 1460 1525 10.0 good 12 0.06 700 30 1440 1360 1510 153215.0 partially incom- plete joining after rolling 13 0.05 1500 25 14501360 1510 1532 31.3 partially incom- plete joining after rolling 14 0.051900 50 1512 1360 1510 1532 25.0 good 15 0.002 700 25 1450 1355 15201535 31.5 partially incom- plete joining after rolling 16 0.002 1900 301524 1355 1520 1535 15.0 good 17 0.002 1300 30 1526 1355 1520 1535 10.0good

As apparent from the table, when the heating processing was carried outunder the conditions satisfying the preferable range of heatingtemperature, although a partially incomplete joining occurred, excellentcontinuous rolling actually having no problem was able to be performed.

Comparative example (1)

A type of steel and rolling conditions are the same with those inEmbodiment (1).

Heating was carried out under such a condition that a meltingtemperature of scales of iron oxide (T_(c)), a solidus line temperature(T_(S)) and a liquidus line temperature (T_(L)) were obtained fromcomponents of the metal pieces and an ultimate temperature of heating Tsatisfied (T_(c)+T_(S))/2>T.

The obtained result is shown in Table 3.

TABLE 3 Content Plate Plate of C width thickness T T_(C) T_(S) T_(L)Draft Result of No. (wt %) (mm) (mm) (° C.) (° C.) (° C.) (° C.) (%)rolling 18 0.5 1200 30 1415 1365 1475 1500 10.0 partially incom- pletejoining after rolling 19 0.002 1000 25 1420 1355 1520 1535 31.3 ruptureduring rolling 20 0.002 1900 30 1430 1355 1520 1535 15.0 rupture duringrolling

As apparent from the table, when heating/joining processing was notperformed under the heating conditions shown in the table, the excellentfinishing rolling was not possible in any case.

Although the embodiments has been described as the cases where mainlythe carbon steel is a target material for joining, it is considered thatthe same effect can be obtained when silicon steel or high alloy steelis applied.

In addition, even if the sequence of joining is changed so that pressingis carried out after heating or heating is performed while pressing, thesame effect can be obtained, and the excellent result can be similarlyobtained by using any other well-known means other than inductionheating.

In this invention, there has been explained about the steel having a Ccontent of not less than 20 ppm, but it is obvious that the temperaturesT_(S) and T_(L) rarely show variations with value close to 20 ppm, andit is thus understood that steel having a C content less than 20 ppm canbe also applied.

INDUSTRIAL APPLICABILITY

1) For heating and joining metal pieces, since an alternating magneticfield whose direction is opposed to that of an alternating magneticfield generated in the metal pieces is produced, corner portions of themetal pieces are sufficiently heated without a fear of melting down in acentral region of the metal pieces in the plate width direction, anduniform heating over the entire region in the width direction of themetal pieces consequently enables a length of the incompletely-joinedportion of the metal pieces which may lead to rupture of the plateduring rolling to be extremely short, thereby performing stablecontinuous hot rolling. Specifically, each reverse magnetic fieldgeneration circuit for generating a reverse magnetic field operates byonly opening and/or closing a switch provided thereto in the joiningapparatus, thus prominently simplifying structure and control of theapparatus.

2) By lapping the inductor over each conductive member provided to eachwidth end of the metal pieces, the magnetic flux directly runs throughthe inductive member to flow the induced current thereto, and a heatingefficiency at the width ends of the metal pieces can be hence greatlyimproved.

3) Since the induced current can flow in the vicinity of corners of themetal pieces more than ever by positively flowing the current having thesame phase with that of the induced current generated in the metalpieces from an external power supply to the conductive members, aheating efficiency can be improved at the width ends of the metalpieces.

4) Since an overlap width of the inductor (magnetic poles) and the metalpieces can have an appropriate value, the temperature can be increasedto a value required for joining in a short time of, e.g., 10 seconds.Further, the same effects can be obtained by adequately securing adistance between the metal pieces and coils for induction heating. Theseeffects can prevent the scale of the apparatus from being enlarged andclarify a preferred positional relationship between end portions of themetal pieces to be joined and the inductor, and the uneven distributionof temperature during heating can be extremely minimized.

5) For performing continuous hot rolling of the metal pieces, sincetemperature rising quantities at portions of the preceding metal pieceand the succeeding metal piece where joining is desired can beindividually adjusted to perform heating and raise the temperature, evenwhen the both metal pieces are different from each other in temperatureor they are of a steel type having different plate thicknesses ormelting points, they can be assuredly joined to each other, thuseliminating a problem such that the plate is ruptured from a joinedportion during rolling to stop the production line.

6) Since phases of a plurality of magnetic field generation inductorsarranged in the plate width direction are controlled, even in the casewhere widths of the metal pieces are changed, the metal pieces can bestably joined to each other.

7) In a method for joining metal pieces by which induction heating isperformed by magnetic fluxes running through the metal pieces in thethickness direction thereof, since a plurality of induction heatingcoils are provided in the width direction of the metal pieces and eachspace provided between induction heating coils are set to be in apredetermined range, even if the metal pieces having a large width are atarget of heating and joining the excellent heating is possible over theentire area of the metal pieces in the width direction. As a result, agood joined portion can be obtained and stable continuous hot finishingrolling is enabled.

8) Since the metal pieces are heated at portions to be joined underpredetermined conditions, joining is assuredly enabled irrespective oftypes of steel, and stable continuous hot rolling can be effected whilelargely decreasing problems such as rupture during finishing rollingwhich is carried out after joining.

What is claimed is:
 1. A method of joining metal pieces wherein: a rearend of a preceding metal piece and a front end of a succeeding metalpiece are heated and the metal pieces are pressed against each other forjoining before hot finishing rolling: a temperature T (° C.) in a rearend of the preceding metal piece and the front end of a succeeding metalpiece is adjusted to be within a range defined by the followingexpression. T _(S) ≦T≦(T _(S) +T _(L))/2, where T_(S): solidus linetemperature of the metal piece (° C.) T_(L): liquidus line temperatureof the metal piece (° C.).
 2. A method of joining metal pieces wherein:a rear end of a preceding metal piece and a front end of a succeedingmetal piece are heated and the metal pieces are pressed against eachother for joining before hot finishing rolling: a temperature T (° C.)in a rear end of the front end of a preceding metal piece and thesucceeding metal piece is adjusted to be within a range defined by thefollowing expression. if T_(c)≦T_(S), (T _(c) +T _(S))/2≦T≦(T _(S) +T_(L))/2, and if T_(c)>T_(S) T _(S) ≦T≦(T _(S) +T _(L))/2 where T_(S):solidus line temperature of the metal piece (° C.) T_(L): liquidus linetemperature of the metal piece (° C.) T_(c): melt temperature of an ironoxide scale (° C.).